Leaky wheels/tires?

[Papa Weeley_imp]

"I dont understand how they just up and lose air without running over something"

The molecules of air and especially the water vapor component in the air dissipate through the rubber over time.

I understand that air escapes the tire casing through osmosis, as well as the fact that rubber is somewhat porous anyway, that impacts on the tire can affect psi, that temperature affects pressure to the tune of about 1 psi per every 10 degrees, that valve stems and cores can leak, etc., etc.
My concern is that 2-4 lbs. per month - at an even rate front and rear - just seems excessive to me. As I mentioned earlier, I've owned about 50 cars, and around 15 or so bikes, and have only experienced that kind of consistent loss when an actual problem was diagnosed.
It's obviously not a huge problem, and luckily I check them on a semi-regular basis anyway; it just seemed weird to me. From the responses here, it seems like this is fairly common with this bike, though, so I'll reel in the concern a bit.