Actually, I've always understood "keepers" to mean something slightly different. When professionals talk about keepers they tend to refer to stuff they are really happy with, and hence the idea that professionals are generally very happy if they can come up with 1-2 keepers per roll. By no means will they necessarily bin the other 34 or so exposures.
Personally, I've stopped counting now how many keepers I get, but if I get one from any given outing, I'm usually happy. But I keep almost all my shots except those that are technically off.
The difference I guess is in the understanding of the word "keeper", and also in the editing. A professional edits far more strictly than an "amateur". Something I still have a lot to learn about
I use a different grading system to most, however as Jed rightly points out those of us who's living depends on our work tend to be far more critical about our work.
Shots are broken down in to the following categories at the first batching.
Shots that are sellable.
Shots I'm not so sure about, may have technial flaws or compositional errors that I want to think over.
Rejects, technical or composition errors that I find unacceptable. These are the images I pay most attention to in the longer term as analysis of these shots leads to a reduction of mistakes that follow a predictable pattern.
A second round of batching the following day normally resolves if the shots in the undecided category make it to good or the circular file.
Overall however I aim to not have more than 6-8 shots per roll for general photography that end up in the circular file, needless to say some days this is unachievable due to either subject material, weather conditions etc.
As for keepers, I probably only print 3-4 shots a month for myself at 8x10 size or larger as I'm seldom 100% happy with a shot.