Hmmm...how to explain this simply.
Okay, number 1 in queue wants 10,000 shares. First seller comes along and sells 7,000 to him - that's one trade. Now another seller for 3000 comes along, sells 3000 to finsih off buyer nimber 1. That's 2 trades executed now but there are still 70 (71 -1) buyers in the queue.
etc etc.
ie 71 trades may not necessarily satisfy 71 buyers.
I hope that makes some sort of sense.