I thought 1000 * 10% should give you 100? That's what the Windows XP calculator tells me. 10% just means 10/100 = 1/10, so 1000*(1/10) = 100.
Something like 1000+12%=1120 works too. In this case (+ or -) you want the % function to take the percentage of the first value, but that doesn't happen when multiplying or dividing by a fraction. I have other calculators that do the same thing, it's clearly by design.
What I've noticed is that, as the maths gets more serious and scientific, the percentage function tends to disappear. My TI-89 calculator has it, but it's buried at the bottom of a menu and barely mentioned in the manual. I also use MATLAB (and its Open Source cousin
Octave) and it doesn't have any percentage functions. (The % sign is used to "comment out" instructions.) I think it's kinda assumed that you can convert basic percentages in your head before you even get there e.g. enter 1200 - 13.5% as 1200 * (1 - 0.135) = 1038