Most of the grey boxes are just because they are from members that don't give many rep points... MOST of them... but if the same person who doesn't give many rep points gives you a negative rep, it will still show up grey. The post that is in question was a negative rep worth -1 points.
and just for the record, members only use half their rep power when giving negative rep. So if you normally give out 20 rep points, then your negative rep is only worth -10.
and just for the record, members only use half their rep power when giving negative rep. So if you normally give out 20 rep points, then your negative rep is only worth -10.