I have a Craftsman lawn tractor with an 18.5 HP Intek Engine (97700 series). The manual says it holds 3.5 pints of oil; however, when I change the oil, I am lucky if I can get 2 pints out. (That is if I drain the oil while the engine is warm.) I know that some oil will remain in the engine; but, shouldn't I be able to drain close to 3 of the 3.5 pints?
It doesn't sound terribly alarming; check the level periodically throughout the season to make sure oil isn't leaking or burning off. Most likely you just can't drain it all.
To begin with the oil filter holds a fair amount. It wouldn't be surprising if there was a pint of oil left in the pleats. Also, if the engine is warm chances are a small amount hasn't drained back yet, so there's a couple ounces.
To top it all off the tractor probably drains from the side rather than the bottom of the engine, which isn't the lowest point unless you parked with the opposite side tires up an incline. On a flat surface another couple of ounces may be pooled and not draining.