How different can a radar gun reading be?

I tested my Bushnell Velocity against a Stalker Sport on multiple pitches at a college baseball game. The same pitches were clocked differently, gaining in difference the higher the speed. The greatest difference (hardest thrower) was clocked at 74 on mine and 81 on the college gun (several times). Would the differences continue to get larger the faster the pitch? If so, by how much? Also, what is the target acquisition time for the Bushnell Velocity, and how important is target acquisition time?

74 mph = 0.55835253937076432934396893569714 seconds
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81 mph = 0.5100998507831674119932555708838 seconds
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0.048252688587596917350713364814 seconds

.046 seconds (Stalker Sport acquisition time) + 0.048252688587596917350713364814 seconds (difference between 74 and 81 mph times) = 0.094252688587596917350713364814 seconds

The Bushnell Velocity acquisition time is about 0.094252688587596917350713364814 seconds according to these calculations.
Furthermore, compared to the Stalker Pro's .01 second acquisition time, the difference is significant, aprroximately 0.08425268858759691735071336481 seconds.