Twitter is making it possible for really popular users to boast about their stats by removing the artificial limit on the retweet and favorites count.
Until now, when the counters reached 50, Twitter would simply display 50+ retweets or 50+ favorites even if there were 100 or 100,000 of them.
That is no longer the case, the accurate count is shown for all tweets. Twitter isn't saying why it changed the behavior now and why it prevented people from seeing the real numbers, but it's probably moot.
It's interesting to notice though that even hugely popular tweets don't really have astonishing numbers, just a few thousands or a few tens of thousands retweets. The change is already live and active for all accounts on the Twitter site.