I was looking at the badges and who had been award certain ones when I came across a user ID I'd never seen before, and found they have a rather interesting profile; in particular no listed posts but 131 reputation, how is that possible shouldn't the reputation from any post not listed disappear?
If you look at their all actions profile tab, you'll see a bunch of post revisions. Their profile lists 16 revisions which, at +2 rep each, add up to +32 rep.
Also remember that it's possible to hide communities in one's profile. That user's public profile shows no other accounts across the network, but they may have hidden accounts; if so, having the 100 rep association bonus is not unreasonable.
At that point, you have 1 + 100 + 32 = 133 rep. The user currently has 131 rep, so this adds up nicely. It could be that one edit has been reverted or something like that which would account for the missing 2 rep; I haven't looked closely at their profile.
In general, see also What is reputation? How do I earn (and lose) it? in the help center.