You are not alone; most business people comment that ACC levy invoices do not make much sense.
Part of the reason for this is the way ACC levies are calculated, some in advance and some in arrears. Often this is not in a sequential order so you might be paying this year's and paying a wash up on a year before last. This is usually because of the way you have reported your accounts; this is also across two or more levy invoices. Added to this the various components to cover the past and future risks ACC have, credits that happen if you have overpaid a portion calculated on a prior estimate.