i beleive if the person who gave you rep doesnt have much for rep themselves, then you just see they tried to rep you but you get no points.
on the other hand if someone with mad rep like a ganja god give you rep then you get massive points.
to sum it up, how much rep the giver has will determine how much rep you get.
ohhh ok have they gave you rep recently? then if so they gave you rep again but it told them they had to spread the love around before they can give you more. so it shows up as a rep but they didnt write anything because they could. gotta spread the love around