You're missing a few important points here, oil is lazy and will go the easiest route, so wil naturally come out of the nearest way out from the supply, if this were blocked say by a tappet, this would mean the pressure would seek alternative routes, ie other tappet holes.
The rear tappets always are the first to suffer from pressure/supply problems.
This is mostly due to the carbonised/congealed oil being pushed to the rear of the oil gallery each side of the head, that is why it is imperative to clean out with a brush and parrafin or similar both oil galleries, some can need a bloody good hefty poke to break through the crap at the rear of the channel each side, normally worse on the exhaust side channel due to the extra heat baking the oil deposits even harder, don't forget the dizzy is turning at half engine speed, and your drill may not be turning that fast, having had the head off etc there will be a time before all trapped air in the head casting will be dislodged, the tappets need very little in the way of lube quantity wise.
tabetha