Center line only?

I've only seen two top line Jackpots on video machines (not my own, but people awaiting payment). One was SuperCherry (I think that's the name) and other game, I don't recall exactly. It was an older game though

Both were on centerline. Even though one of these is 15 lines and maybe the other was 20. Or maybe I should say number 1 line which is not always center but usually up or down slightly of center.

It would be possible for these two machines to award top jackpot on one of the diagonals but since I haven't seen it made me wonder if most are programmed only for centerline top jackpot. As an aside, reel slot machine usually only award centerline for top jackpot too even if they have multiple diagonals.

Is this a standard way to design for some reason, or just a custom? Or is it even true?

I have thought the same thing, but my observances are heavily influenced from playing VLT machines which may act differently from class III. Even my observance through millions of spins on one game still show the majority of 10,000 credit wins to be weighted towards the lower lines.

My observances as I remember them.
line 1-1 time
line 2-0
line 3-1
line 4-5
line 5-3
line 6-2
line 7-1
line 8-1
line 9-0
I am sure about line 1,2,3, 8, 9 for a fact and the rest are definitely proportionate to how I remember seeing them though may be understated. It has been a long time. With all times but the first line, I had seen them, but not won them if it matters. It has always made sense to program the simpler lines to have the big hits, because they are simpler to understand and prettier.

I did have the beans on super cherry on line 12 in the bonus and five Easy Riches on line 13 playing class III.