I have 2 white MCC approved plastic guages and one metal guage made to MCC measurements that I can check bats with....they are not as consistant as you may think. I can get my pro bats to pass through the guages differently.
Umpires are all also different, they don't all pass the bat through the same and any form of resistance on the guage (which can be from the guage not being passed over the bat squarely) is met with an attitude of the batsmen being a law breaker rather than trying to fit the bat through.
The guages are also rediculous.....I've had a scoop not pass through.
In summary....the guages are not consistent, and neither are the umpires.