Math Functions broken?

I've been trying do something like "input.Sub_Total = (input.Sub_Total + 2.25)" but this doubles the number to equal 4.5, and if applied twice, it makes 6.75, etc. Shouldn't it work, as all I'm really telling it to do is "x=x+2"? If there is any easy answer (and I'm sure there is), please tell me.
pxl_buzzard