It's an ESTIMATE of points allowed per 100 possessions, based on his formula. His formula gives you bonuses for things like blocks, steals, etc. and it takes away from you for things like fouls, etc.
The stat on 82games.com is just a straight up stat -- how many points were allowed per 100 possessions while the guy was on the floor. The stat on basketball-reference.com is an ESTIMATE of points allowed per 100 possessions created by Oliver's formula (which accounts for things like blocks, steals, fouls, etc.)