The speed monitors are notorious for being inaccurate at a lot of places.
When their working they are pretty accurate for the speed at the pins (For your "off hand" speed the rule of thumb is to add about 3 mph to the monitor speed.).
The thing to really use them for is to watch for changes in speed. Even if they don't tell the correct speed, they will still show you if your varying your speed from shot to shot.
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