accumulation/distribution graphs on the charts are not scientific - they're generalized indicators of what *appears* to be pressure on ask vs. pressure on bid at close of trading for the period indicated on the chart. But they're not foolproof, and don't tell the whole story for the period's trading as a whole. (For instance, you can have a million shares worth of asks being filled all day long, then in the last 30 minutes get 10,000 shares filled at the bid as the last of the daytraders close out, and if the day closes lower than the open, the entire daily volume is counted as "distribution" on a daily chart... there are other flaws as well, but this is probably the most visible.)
that said, don't you think short covering would show up as pressure on the ask (indicating accumulation)?