I'm wondering how reliable Credit Karma's score is? I subscribed to Discover's Credit Score Tracker, and it was completely bogus -- almost a hundred points off. I know Credit Karma pulls directly from TU, I just don't know how accurate it is.
The CK score is TU's 'New Account Score' (or something very similar; it might be scaled idfferently), which is a score TU sell to lenders to use for making new account decisions. (I don't know if anyone actually uses it, or not, though.) It's TUs guess on how likely you are to default in the first 24 months of a new account.
So, it's not going to track your FICO score terribly accurately, because it places more emphasis on recent history, and discounts older history. It doesn't include closed accounts, for instance, because TU doesn't think an account that's been closed for years is relevant to how likely you are to pay back a new account. It appears to count utilization higher than FICO, appears to have a much higher new account penalty, and maybe counts account age as more important. It also really hates inquiries. (TU's actual product dedupes mortgage and auto loan inquiries; CK doesn't seem to.)
CK's reporting of events (inquiries, limits, usage, etc) is pretty accurate. So it's useful for that, if not for the score's predictive value. Of course, I'm not so sold on the FICO that consumers can get for predictive value, either.
Outside of the mortgage industry, there's real competition for customers, and there's a lot of models that go into a credit approval decision, which include lots of things that aren't in the classic fico scores. Even if the score that they give you is the straight fico score, they don't just say "719 no good; 720, good". They said "720, and won't make us any money in interest, not a fit for this product" or something like that.