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Some lenders use their own internal scoring model, which basically is unique to them and would provide them with a different score than would be found anywhere else. I'm not sure what CO is using these days.
There are only 3 factors that go into a credit score. The scoring model used, the bureau data used and the time at which the score was generated using those first 2 factors. If you are seeing a different score, one or more of these factors is different. If it's an internal scoring system, then the model is indeed different. Different bureau data is also a common difference. Time is tough to gauge at times, as unless you are checking your score daily it's often difficult to pinpoint when the change happened. For example, you may get your scores from myFICO on the 1st of the month. Then on the 15th of the month, you app for something and are provided with a score that's different. If the scoring model and bureau data used are identical, it means that something in those 2 weeks changed on your credit report. Unless you are monitoring your report/scores daily, it would be difficult to know when during those 2 weeks that something changed that impacted your score.
Great reponse by BBS.
As a followup, your credit card balances can cause your scores to vary a lot. If you make sure that your cards are all reporting $0 except for one (the remaining card reporting a small balance) that will make any particular score more stable and less likely to fluctuate in a big way from week to week.
This All Zero Except One strategy (AZEO for short) won't help you build your score over time, so many people don't do it much of the time. But it will give you as many extra points as you can get in the short term run up to an application for credit. And as I say it will make any particular score more stable, less prone to swings.