It's important to remember the stats they use at Wages of Wins are DESCRIPTIVE, not PREDICTIVE. The stats they use are raw.
That is, these season predictions are based on what the stats for every player/team is as of the All-Star Break. All of our efficiency stats are low because of the losing streak. If you had calculated these same stats at the end of our winning streak, they would be different.
While it is worthwhile to look at these stats, you have to remember they do not take into account things like injury, suspension, etc. etc, nor do they control for good players going through slumps (see Pierce, Paul). If our guys aren't playing well, our stats will be terrible. The fact that our core group hasn't been entirely healthy yet, and Pierce and Rondo have been inconsistent so far, there is reason to believe that our team will improve in the second half of the season.