I noticed this years ago and the OP is correct...

ZC calculates the vat on individual items and rounds it, then adds the rounded amounts. When the cart has ONE item (or perhaps just a couple), this is not serious as the vat total will usually be correct.

But when a cart has many items, the vat total can be out by several pennies!

I believe this may have something to do with the policy of sales tax calculation being different in the USA, where tax can be added to individual items and rounded, before it is finally totalled up...

... but in the UK, vat policy is to apply the vat to the transaction TOTAL.

This may be complex to resolve, as it may require a completely separate taxation module for markets where tax is applied to the transaction TOTAL, rather than to individual items.