[bitfolk] Small amounts of RAM missing - any ideas?

Hello, We had a support ticket earlier where a customer with 480MiB RAM is only seeing 464MiB of it as reported by "free -m". Now, I suspect this is not a new thing, and has only been noticed by them because the ongoing RAM upgrades made them take an interest in how much RAM they actually have. I've had a look at many of my own (Debian) VMs and they all see exactly as much RAM as they have been assigned. I've mentioned this on IRC and a few other people also see slightly less RAM than they have been assigned. Most so far appear to be using Ubuntu, though there are some with Debian who are missing a few MiB of RAM too. It's not a big deal and I'm not asking you to let me know if you do or don't see it too (there's currently nothing I can do about it). I was just wondering if anyone knows where it goes? Perhaps some kernel feature that eats up to 20MiB RAM, which I don't have enabled? My own kernels are 32bit PAE, so I don't think it's a PAE thing. Cheers, Andy