Bluetooth is watch initiated, and only turns on 4 times a day (or when the user explicitly requests it with a button press). The pedometer is probably a special chip.

You should compare this with the F-91W that gets 7 years on the same battery, and hobbyist hardware mods that get years out of the same battery running ARM chips.

To be fair, the hobbyist hardware mod variants that do step tracking only last for days, not years AFAIK.