a hot roll means a lot of dead elite infantry.I think S3 Lethal Strike is intended to represent that it's unreliable; against a naked R3 target, it does less than S4 AP1 would, but sometimes it just... gets lucky.
I mean, bear in mind, the other OK artillery weapon is a cannon; OK are allowed top tier artillery weapons, and the Scratapult is still no Plagueapult. Getting AP6 on 6's is still worse than getting it on all of them.
But yeah - OK gets good shooting. They just do. They're an elite army with some issues hitting (low Aim, medium OS, no special rules helping hit) but good combat and good shooting. It's their identity.
If my S2 WS2 swarms get enough 6s they can waylay a dragon in a turn of hitting.
Its what keeps the game fresh, if matchups had the same outcomes every game it would become incredibly stable, very quickly.