Some advice for our OP:
* Spend some time working with the folks on this thread understanding the difference between (a) the three bureaus (TU, EQ, EX) and (b) the algorithms that produce credit scores.
(A) are three different repositories or warehouses that store information about your credit history. By themselves they are just a list of accounts -- they are not scores.
(B) are computer programs that take the mess of data in (A) and turn it into a single 3-digit number (typically between 300 and 850). Some of these computer programs are made by a company called FICO, some are made by another company (its score is called Vantage), and some are made by other companies still.
It's really important that you make sure you understand the difference between (A) and (B). One of the things that will help you understand is obtaining your full credit reports for each of the three bureaus in (A). You'll likely see that the information in each report is not quite the same.
In your case you had a score drawn on TU (by Cap One) using some kind of scoring model (but we don't know which one yet). The other score (Discover Scorecard) was drawn on EX and used the FICO 8 Classic model.
Because the scores (B) were generated from different data (A) you should expect that the scores were different.
Even if the scores were both drawn on the same bureau, we don't know that the scoring model was the same -- in fact it very likely was NOT the the same. MY guess is that Cap One used an old FICO model that is widely used for mortgages. If you were to tell us the exact wording of the denial letter we might be able to confirm that, especially if it says in the fine print what the range is of the score Cap One used.
Even if the scores were drawn on the same bureau (they weren't) and used the same model (probably not) they were likely generated some time apart. Even if the scores were generated as little as one day apart that could result in different scores.
Some practical advice for you:
* Work on making sure you understanding all of the conceptual stuff I just went through. Understanding those big ideas will be essential in your rebuilding process. The other folks here can help you.
* Pull all three reports from annualCreditReport.com (TU, EQ, EX). Look through them very carefully and find out what all the bad things each one is saying. Also look at the balances you owe on accounts.
* Work with people in the rebuilding forum at pulling up your scores. This may take a long time and you'll need to talk to them about what is on your reports. Your scores are so extremely low that you should not be applying for any credit products at all (loans, credit cards, etc.) for quite a while -- not until all three FICO 8 scores are at least 640. An important exception would be secured credit cards. Again, the folks in he Rebuilding forum can guide you on secured cards. Make sure they grasp how low your scores are, however.
* Use the $1 trial at Credit Check Total once every three months or so to track your overall progress. The free monthly EX score at the Discover Scorecard is also fine, but CCT will give you all three bureaus (reports and FICO 8 scores) drawn on the same day. Also use free tools like Credit Karma if you want more frequent reports, but be sure to ignore Karma's scores and any advice it gives you.
* Finally, work to get all credit card debt paid off except one open card reporting a small positive balance. All cards at zero except one (called AZEO here) will not cause your rebuilding process to go faster, but it will optimize your current score (get you every available extra point) which is especially helpful before you do anything that involves a creditor pulling your report or score (that could include applying for a secured card, joining a credit union, asking Comcast to hook up your cable, etc.).
