I can't do that, because the user explicitly typed in "4.1m" in the textbox, which means the number that the computer has to interpret is "4.1", followed by a "m".

Because of the way computers store numbers, 4.1 is really stored as 4.09999999999999964472863211995. After multiplying by 1m and rounding down the result of the multiplication _towards_ 0, this becomes a final value of 4099999.

Is this now simple enough to understand? Basically, numbers with decimal parts cannot be stored _exactly_, unless it can be decomposed and added up by fractional powers of 2. (i.e, 0.5, 0.25, 0.125, etc). Obviously, if the user actually typed in "4100000" instead of "4.1m", this wouldn't be a problem.