There are many possible reasons. If you have derogs (lates, collections, etc.) the mortgage models treat them differently than does FICO 8.
For example, if you have a collection that is for under $100 (paid or unpaid) FICO 8 treats it as if it was not there at all. The mortgage models in contrast would penalize you for it, possibly quite harshly.
If your only derogs were very old Day 30 lates, FICO 8 would be likely to assess only a small penalty -- whereas the mortgage models would be much less forgiving.
Have you pulled all three credit reports and gone over them carefully? Reports are different than the scores.
Also, can you tell us how many credit cards you have? Even better would be for you to list each one with its balance and credit limit, like this:
Card 1. Balance = ____ Limit = ____
Card 2. Balance = ____ Limit = ____
Card 3. Balance = ____ Limit = ____
etc.
If you do that we may be able to suggest a strategy that will help.
