Never seen the like.

Happens from time to time. Especially if you have older meters that are harder to access, they may not take a reading for a while, bill you by estimation...
Also, and it seems that this may have happened to you, all water is metered at least twice, building and apt. If they can tell that some of the meter are not reading properly,(by the total of the individual meters being higher than the main meter) then they will change the meter and refund what they estimate you are owed.