Well, for one thing the rate of reduction you're talking about is over 24 hours, not 2 - the tube doesn't stop being porous when you get off the bike.

But more importantly, it isn't linear. The higher the pressure, the harder the air molecules are being forced through the interstices of the tube. I'm not sure what the curve looks like exactly, but you must have a rough idea of it already - don't you ever take a day off riding? On the second day, the pressure will be 70-75, on the third day it'll be 70, and if you store the bike long term it'll be months before it's completely flat.