round using round function vs the decrease decimal

Hi
Let say I have A1 =100.1234
When I select A1 and then Home-->Decrease Decimal and I click 2 times, I see the A1 = 100.12 but when I click on cell A1, I see it is 100.1234 in the formula bar.

Now if I do this B1 =round(A1,2), then I see B1 = 100.12

If I subtract A1-B1 then I see 0.00 ! Why 0.00 should not be 0.0034
because A1 = 100.1234 and B1=100.12

Thank you.