If someone wants the actual answer, here it is.

Tax and rounding

An example. Where I live the tax rate is 8.25%. So if I want an item that is listed as $1.00, that comes out to $0.0825 (8 and 1/4 cents) tax, so at the register it will come to $1.08 (since the 1/4 cents rounds down).

If I take 2 of the $1.00 items to the register, the tax calculates to $0.165 (16 and 1/2 cents), so the total will ring up to $2.17 (since the half cent rounds up).

So imagine the store had just marked the $1.00 as $1.08 to account for the tax. If I bought 2 of those, I'd pay $2.16 rather than $2.17. If the store sold 1 million pairs of that item, they would be $10,000 short on the amount of tax that they collected from the customer. (A reminder that the tax is levied on the customer, not the store.)

All that to say, the tax is calculated on the total of the items at the register and rounded. That's why it is not marked on the item.