I have a 25 year fixed-rate mortgage as well, it's been 3 1/2 years now, and my last statement showed remaining amortization of 17 years and 24 weeks. I never paid attention before, but last year's statement for the same date showed 18 years and 24 weeks as well.
I calculated it myself using the current (fixed) interest rate and came to 17 years and 39 weeks, somewhat more than the bank's figure. Either way, it's several years short of the target (I should have roughly 21 and a half years remaining). I called and asked my bank about this last week, and they couldn't figure it out; they told me they'd call me back... that was a week ago. As far as I can tell, my weekly payments weren't set up properly and are somewhat higher than they should have been.
I tried a variety of things to get 17 years and 39 weeks down to the bank's 17 years and 24 weeks but couldn't- I have no idea how they came to that figure either.
The fortunate side : I can make payments up to twice the regular amount without any sort of 'overpayment penalty' , so the fact that I'm ahead won't cause me problems in the long run.