Partly because it was designed not to work precisely in the normal way we might think of precision. I think.

Huh?

Yes folks. Welcome to this excellent piece by Joel Spolsky from way back in 2002 about Leaky Abstractions.

I don't understand it. But it is elegantly written.

What a super writer Joel is. Have a look at his analysis on how to price a piece of  new software. Go for the top end of the market? Almost give it away? Somewhere in the middle? All of them at the same time if you can get away with it? 

Eeeek - too many options.