what is a 7 percenter?

Answer 1

A left-hander, a lefty, a southpaw, a mollyduker. Seven percent of the population in the U.S. is left handed.

Answer 2

It may be similar a a phrase we use called a 20 percenter.
The rule is that 80% of the work done by your staff, comes from 20% of your workforce.
In other words, 20% are grafters/experienced, 80% are lazy or inexperienced.
A 20 percenter is then one of your key players and should be looked after.