I think you're right about the calculations being fun. My son tells time by counting fives from 12 though he has known the hand placement for o'clock, half -past, and quarter-til since he was little. One time he described how he had a computer screen in his brain where he can see the numbers turn different colors when he adds them, and I think he described some sort of imagined abacus. I'm a hugely visual learner, so I related it to seeing words in pictures before I learned to read ( I always saw Tennessee as two tennis rackets crossed and Kansas as the female version of John Denver)...I'm odd, too!
