Actually, many times checkusers run checks of accounts whose edits are stale.  For one thing, it is simply easier (lazier) to click the "check" button and let the system tell me whether there are any results, rather than first checking contribution history, especially when checking a lot of accounts.  For another, the checkuser tables include deleted edits and the sending of emails through the special:email function, neither of which shows up in a quick check of contribs (although deleted edits would show if I looked at both the live and deleted contrib pages).  As far as the oversighted edits, I'll cover that in email with Giano. Thatcher