I feel your pain - we had exactly the same thing one month last year, when receiving the annual bonus made the percentage shoot up, as the computer multiplied that monthly amount by 12 and deducted tax as if we were going to earn that much throughout the year.

We asked on here, hubby's HR, friends, etc, and everyone shrugged and said it was just the system and it would all come out in the wash at tax return time. Which is slightly less than reassuring, given we're still waiting on the tax number to file the return for flippin' 2008.

kodokan