They ran different diagnostics, or one ran none
A design follows from how the soil under the slab communicates, and that is measured. Two people who measured different points, or on different days, have different pictures of the same house. One who did not measure at all is working from the visible building.
What to ask. Ask each what diagnostic they ran, where, and what the reading was. The answers are comparable even to somebody who has never seen one before.
What would settle it. A communication or pressure-field test performed at the same points, with both parties told the result.
They assumed different things about what is under the floor
Nobody can see the sub-slab material or whether drain tile runs all the way round. Two professionals make different working assumptions about the same hidden condition, and each designs correctly for the assumption they made.
What to ask. Ask what each is assuming about the sub-slab material and the drainage, and what they would do if the assumption turned out wrong.
What would settle it. Physical investigation — a core, a camera, or the original construction drawings if anybody still has them.
They read your measurement differently
A short-term result in mild weather and a long-term result mean different things, and a reading close to a threshold can fall either side of it on a different week. Two people can look at the same figure and hold genuinely different views about how firm it is.
What to ask. Ask each what they think your number establishes, and what they would want before acting on it.
What would settle it. A longer measurement, or a second short-term test under controlled conditions, depending on your timeline.
They are quoting for different work
Verification testing, electrical connection, permits, sealing, and making good are all sometimes in and sometimes out. Two totals that differ by a large margin often describe two different jobs rather than two prices for one job.
What to ask. Ask each what is included and what is most commonly added later. Compare the scopes before comparing the numbers.
What would settle it. An itemised proposal from each, listing inclusions and exclusions line by line.
They have different tolerance for being wrong
One professional specifies for the worst plausible condition; another specifies for the likely one and returns if it underperforms. Both are defensible positions about the same house, and they produce different designs and different prices.
What to ask. Ask what happens if the system underperforms — who returns, at whose cost, and how quickly.
What would settle it. Nothing settles this in advance. It is a genuine judgement, and the guarantee terms are where it becomes concrete.
They have worked on different housing stock
Somebody who has spent fifteen years on post-war basements and somebody who works mostly on rural crawl spaces have both learned real lessons, from different buildings. Faced with an unusual foundation each reaches for what has worked for them.
What to ask. Ask how many houses like yours they have worked on, and what surprised them on the last one.
What would settle it. Nothing settles it, but the answers tell you which of them is on familiar ground with your particular building.
The house was not the same on both visits
Weather, whether the windows had been open, whether a fan was running, and what has been done to the building since the first visit all change what somebody observes. Two accurate observations of a building in two different states will differ.
What to ask. Ask when each visited and what the conditions were. Compare that against what you know about the weather and the household that week.
What would settle it. A visit under the same conditions, or a measurement that spans both.
One of them has not explained their reasoning at all
This is the one difference that is not about the building. A proposal that arrives as a number and a method, with no account of why this house needs that method, may still be right — but you have no way to tell, and neither does the other professional.
What to ask. Ask why this approach for this house rather than another, and what would change their mind. A reasoned answer is not a guarantee of correctness; the absence of one leaves you with nothing to weigh.
What would settle it. Asking. Most of the time the reasoning exists and simply was not offered, because it is obvious to somebody who does this daily.
The one difference worth treating differently
Every reason above is about the building or about a judgement. There is one that is neither: a proposal that arrives as a method and a number with no account of why this house needs that method. It may well be the right proposal — but you have no way to weigh it, and neither does anybody else.
That is a missing step rather than a bad person, and the fix is usually just asking. The reasoning normally exists and simply was not offered, because it is obvious to somebody who does this every day. The question builder puts “why this approach for my house rather than another?” in every list it makes, for exactly this reason.