(Another reason to be able to choose FGs in the trainer--bug checking.)
So, I recreated the bug. I had 11 Ball Lightnings on the field, 2 Boneyards and a 7-bone Bonewall. The end result was 12 Skeletons and a 19-bone wall. The delays with the hits, caused delays in the summoning of the skeletons, so that each Ball Lightning only produced 1, except for either the first or last one which produced 2. The Bone Wall followed the Skeleton's lead and only produced 12 instead of 22.
With 2 Boneyards:
1 Ball Lightning > 2 Skeletons
2 Ball Lightnings > 2 Skeletons
3 Ball Lightnings > 4 Skeletons
4 Ball Lightnings > 4 Skeletons
5 > 6
UPDATE: Verified in real game vs AI3. Every second Ball Lightning when going against the delay shield does not trigger death effects--Boneyard, Bone Wall and Vulture.