[TYPO3-shop] unexpected tax calculations

[John Nicholas]

I'm using 2.7.0

I'm in the US so taxes are not included in the prices. The sales tax for 
my state is 8.25%

I have plugin.tt_products.shipping.TAXpercentage = 0 and 
plugin.tt_products.payment.TAXpercentage = 0

I have 2 products in my basket, 1 at $28 and 1 at $80. This is a 
subtotal of $108 and my taxes show as $8.91 which is correct. I then add 
another of the $28 items which brings my subtotal to $136. My tax should 
now show as $11.22, instead I see $12.12. If I now add another of $80 
items (for a basket of 2 at $28 each and 2 at $80 each) the subtotal is 
$216, the tax should be $17.82 but shows as $19.63.

Any guesses what could be throwing these values off?