I think it's kind of like population density...just because a lot doesn't come out, there still might be a good volume of healthy sperm. In contrast, if "a lot" comes out, it may be a lot of ejaculatory fluid and the same amount of sperm.
When they run the test, volume is one of the things they measure. I don't know how relavant it is, but they do include it in the test.
i mean to say volume of semen rather than sperm. sorry :) i'm sure most people could figure out what i meant anyway...just in case though. :)