Because not 69 nor 73 :D It's arbitrary number. When you resize window, it throws events once per 5ms - 20ms. Some times few events that trigger `rebuild` are called one after another. User won't notice any thing that happens in 50ms interval. But I think 50ms is to short to prevent 30 rebuilds per second (that may happen in some situations like filtering ng-repeat). I say that 100ms would be the best. But that's too long for user. So I decided to choose something in the middle. 75 was to obvious :)