As I understand it, the front bearing is not intended to be the front seal. The front seal is supposed to be the close tolerance fit between the engine case and the crank. A thin layer of oil is supposed to fill the gap and make the seal. To keep the front bearing lubricated there is a small weep hole between the case and the front bearing that allows crankcase oil to ooze into the bearing. The dimensional tolerances of these features are critical and if they are not correct you have an engine that doesn't transition well and/or doesn't idle well. Certain brands of engines seem to have more variability in their manufacturing, hence more bad samples in the bunch.

I may be totally wrong.