How are my scores so off?

I had been using CreditKarma since late april and had seen my scores raise steadily since then. I paid off a few old collections and disputed them directly through their website and two were taken off within a week! So as of today at CK my scores are 646 and 638. This had me super excited and thinking I was that much closer to my goal of buying a house in the next year. Well, haha, not so fast. I signed up [at myFICO] and got my three scores: 577, 583, and 603….. SUPER DISAPPOINTED. HOW ARE THEY SO OFF?! But with using myfico now i see that I have another past collection that I couldn’t see with CK and I will be taking care of it asap. I like CK only for the fact that the disputes were so easy and that at least I could see some movement in my scores, even though they are misleadingly high, AND I got two CC based off their recommendations.