in this image, we are looking "up" the transfer pipe disharge passage in the block that goes up to the main passage to the oil filter.

If you look "down" the hole, you will see the step there where the drilled hole is not centered on the main passage. It actually would not be "centered" on the main passage; but rather slightly offset. The problem is that the drill was not sent in deep enough to make the intersection a full one. If the bit had gone in another quarter of an inch, the flow would have been much less restricted. By not going that extra little distance, a step was left, reducing the size of the intersection of the two passages by nearly half of what it could have and should have been.