The 920xt takes a little distance to get the pace adjusted. It's kind of like it picks a point a certain distance behind you (100m or so) and uses that for distance. Also, going around the curves affects the accuracy, depending on where it gets the GPS reading.
One way to get accurate splits for a lap is to start at the 50 yard line (or middle of the straightaway) and check your time at each quarter of a lap, at the goal posts (or middle of the curve) and at half a lap. For example, if you want a 6 minute pace, you should hit the middle of the first curve at 22.5 seconds, 45 seconds at half a lap, 67.5 seconds in the middle of the second curve, and 90 seconds on the lap. You can hit the lap counter (using manual laps) and display interval time if you want to see the same numbers for every lap. It's probably good enough to check the time every half lap and forget the quarter-lap times.
One way to get accurate splits for a lap is to start at the 50 yard line (or middle of the straightaway) and check your time at each quarter of a lap, at the goal posts (or middle of the curve) and at half a lap. For example, if you want a 6 minute pace, you should hit the middle of the first curve at 22.5 seconds, 45 seconds at half a lap, 67.5 seconds in the middle of the second curve, and 90 seconds on the lap. You can hit the lap counter (using manual laps) and display interval time if you want to see the same numbers for every lap. It's probably good enough to check the time every half lap and forget the quarter-lap times.