I'm not sure this is true. For example, consider the following scenario at $200 USD per bitcoin: 5 people pooled mining capabilities, and mined a single bitcoin. They could then split that, and effectively pocket $40/ea (or 0.20 bitcoins each). If you mine solo, is it possible to mine a partial bitcoin, rather than increments of a whole bitcoin?
Right: mining produces whole bitcoins, and does it infrequently. That's why pools exist; people weren't willing to wait months to randomly win the bitcoin lottery. (Maybe that's a design flaw!)
Most pools pay proportions of their actual winnings, not their predicted ones.