The author asks for someone to contact them with the answer to this question -- and I'd be interested as well:

A small mystery remains: if the statistics

  from /proc/*/statm which are used by htop

sync every 64 page faults, how can 1.5 MB of error accumulate? Syncing every 64 faults suggests that the maximum error would be 4 KB times 63 faults = 252 KB of error.

(sorry for weird formatting, wanted to include my comment around what statistics he's referring to, but asterisks can't be used unless preformatted AFAICT.)