(1) When you're locked, the stats are skewed sometimes. This fixes itself after you unlock the account, not immediately, but in a few hours.
(2) Twitter suspends accounts continuously, suspended accounts are getting removed from your follower list obviously.
What you can do is make a list of your followers and compare it each day. So you'll see exactly what happens, whether those accs you're losing getting suspended or something else. You can use a tool like this:
http://www.quickdiff.com/ to compare two lists and see the difference.