Make sure a "diff" number is displayed before mapping. Like Shane said, one direction only.
The "diff plus a number, like "Diff 8" shows the number of land based GPS stations ZO finds. ZO uses space based and land based signals to be accurate to one meter (39.2"). Diff stands for differential. If you start mapping before they are displayed the accuracy suffers. This corrects satellite drift.
If it beeps about the same distance before in one direction as past the buoy in the opposite direction, that's acceptable. It should only be off by one meter at most.
All this is based on properly working pucks.