It is very clear due to the price unit issue.
the first one is = 83 per 100 qty
and 20 % is the tax
the 83*.2 = 16.6
then for per unit
16.6/100 = .166 (here it is getting rounding off)
rounded off to two digit 0.17
and hence for 100 pcs 0.17*100
In the second scenario it is .83
here it is direct and hence
the unit value is 0.83
so
and 20 % is the tax
the 0.83*.2 = 0.166
but as a final value system picks two digits and makes it 0.16
better apply rounding off rule in the final subtotal , so the issue shall resolve , even if any round off issue come it wont be greater than -0.4 to +0.4 for a innvoice
This is a logical conclusion , since understanding is based on screen shot presented