It's because it goes to 12 on one hand, then 5 on the other, and then if you go higher it does 12 again then 5 again. You can think of that as going to 60 on both hands, then 60 again, rather than alternating 12 and 5.

e.g. 12×5 (60) seconds, then 12×5 (60) minutes, then 12 hours

Really it's an alternating base 12/5 number system though.