If a particular objective often is succesfully defended for the full round with no chance of moving on then the average completion time of that map will most likely have a high standard deviation. In others, even with a 50/50 win loss rate for attackers, there’s a wide range of completion times for that map. Lots of steamrollers, lots of full holds.
The alternative would be having SD to ‘smooth’ out the map flow. It would decrease the range of map completion times but it would also make the map feel repetitive after a while. The matches will start to look like each other.
It’s not an easy thing to get right. Ideally you want lots of different ways to win or lose a map, avoid the frustrating dead ends while at the same time retaining that win/loss ratio.