Congratulations on the improvement!
You know, that's not surprising that you can improve significantly but still miss a fair share of questions in total. The GMAT grading system is "adaptive", meaning that you get harder questions each time you answered the previous one correctly, and "easier" questions when you've answered incorrectly. So if you're getting enough additional questions right to merit harder subsequent questions, you're probably earning enough points and seeing hard enough questions to justify a significantly higher score. To understand the scoring algorithm more, check out this article (and the one linked within it): http://www.veritasprep.com/blog/2011/09/gmat-tip-of-the-week-think-like-the-testmaker/
As for the practice test reliability...they're all decent but I wouldn't take those scores as pure gospel. Seeing improvement is very promising, but even more important is tracking how you're doing qualitatively. Ultimately the only score that matters is your test-day score, so I'd focus on learning things like:
-How are you pacing yourself?
-Which mistakes do you find yourself making frequently?
-Which question types are taking you longer than they should?
-How are you holding up, in terms of fatigue/stamina, toward the end of a long test?
-When you're rushing or lack focus, how are you having success refocusing?
Those items will help you improve, and improvement is the name of the game...like I said, I think the tests are pretty good as far as tracking, but tracking is much less important than learning/improving, and I do think the tests are great at giving you opportunities to analyze your performance and find items for improvement.
Keep up the hard work!