It originally worked the way you describe. The reason for the change is described in a very old post from early in the Steem era (perhaps someone can find it). The idea is that if you vote 20 times per day with a default or 100% vote weight instead of 10 times per day, your votes will automatically end up being about half as strong each, rather than the first 10 votes burning all your vote power and the next 10 doing nothing (this is an oversimplification as the window is actually 5 days, but you can get the idea). It was to accommodate people at different levels of activity without requiring micromanagement of vote power.