Eight hits has been known as the cap for a weaponskill or one melee round for a very long time. So, it's very possible that eight hits was also set as the cap for one ranged "round" a very long time ago before it was possible to even approach eight hits much less exceed eight hits.

On the bright side, most of the +1 shot items other then Hunter's Bracers +1 are made in the place of something else or downright bad, making this a very minor issue in terms of performance.