well, let me see if I can explain this.
the cylinders fire at 180 degrees, so... the left will fire, then the right will fire, then the left... if you have dual carbs, air is being pulled through one carb for an instant, then the other for an instant. both carbs are not pulling air at the exact same moment.
a single carb is pulling a constant flow of air through the carb, however it doesn't feed both cylinders at the same moment, due to them firing at opposite intervals, therefore supplying a blast of air on demand.
Don't know if that is a perfect explanation, but that should give you the jist.