I think Microsoft had far more urgent problems to sort out with Vista than the calculator.
But I'm not following your math here: 1000 * 10% is 100,000, not 10,000. (10% of 1000=100. 1000*100=100,000).
So your first answer should have been 100,000 then pressing += adds whatever is in the answer window to itself 100,000+100,000=200,000. Pressing += again should give you 400,000, and so on.
The calculator in W7 works exactly the same way BTW.