confusing Artillery overload score

[Imp]

Well done for taking the time to look at it, there was obviously something wrong when 9 offnap tubes represented 37% of his force with a force that size.

Unless you adjust your force I would expect more of the same in the future, your force is very artillery heavy & quite large. Not going into detail but with that combination I would therefore expect the AI to spend a lot on offmap artillery.
I would stick to a max of 15% in your core, use your support points to buy more for an assault if you need them.