Check the rep on those users -- are they below 15? More importantly, were they when your answers were marked as best? "New" users -- that is, users who are below 15 rep -- can't vote, even on their own questions. Likely they wanted to upvote you (they figured out the system enough to mark an answer, after all), but they were simply not allowed to.

A quick glance at your SO profile's lowest-voted answers seems to indicate that these are in fact very low-rep users. I have no problem believing that a user who now has 351 rep was below 15 on February 15, for example.

Certainly a user who currently has 23 rep was probably under 15 4 hours ago. :)

This guy currently has under 15 rep.

(Note all times and reps are as of this posting. I have no intention of keeping them current. YMMV.)

Check the rep on those users -- are they below 15? More importantly, were they when your answers were marked as best? "New" users -- that is, users who are below 15 rep -- can't vote, even on their own questions. Likely they wanted to upvote you (they figured out the system enough to mark an answer, after all), but they were simply not allowed to.

A quick glance at your SO profile's lowest-voted answers seems to indicate that these are in fact very low-rep users. I have no problem believing that a user who now has 351 rep was below 15 on February 15, for example.

Certainly a user who currently has 23 rep was probably under 15 4 hours ago. :)

This guy currently has under 15 rep.

(Note all times and reps are as of this posting. I have no intention of keeping them current. YMMV.)