While watching a New York Mets game early this season, before one plate appearance the broadcast graphics displayed that catcher John Buck had a batting average (AVG) of .400 and an on-base percentage (OBP) of .396. Several games later I saw a similar phenomenon for Buck, whose OBP was again several points lower than his AVG. (In other words, the stats presented were very likely to be actual and not an on-screen typo.)
How is this possible? By my understanding, the two are calculated as follows:
AVG = (hits) / (at-bats)
OBP = (times reaching first base) / (plate appearances)
It would seem to me that the conditions by which a plate appearance does not count as an at-bat would always result in the batter reaching first.
What could have happened to cause Buck's on-base percentage to drop below his average?