I can't tell if you're joking, but when you're counting a quantity, you don't start from 0.

Why not? Is having zero interfaces not distinct from having one interface?

It is. But when you have one interface, that interface can be given the number 0. So you have a total of 1 interface, even though you've only counted up to 0.

Edit: I misunderstood. I'll leave the comment up. But I originally interpreted the story as meaning that each interface needed to be assigned an unsigned 16-bit id, which allows for a total of 65536. That was just inference on my part though. It literally says that more than 65535 are not allowed.

Quantity as in great amount? Either way, sounds like the worst kind of premature optimization to save one bit's worth to lose the ability to ever have the quantity be zero.

I suspect it is a play on the 0, 1, infinity rule.