Every month it won't affect 95% of the customers, by definition.
My guess is they won’t be doing an actual analysis like that, they just figured out where the 95% number is, and that’s where they will start throttling.
Interestingly, this means that early in the billing cycle, they can still bog down the network, and then near the end of the billing cycle, suddenly the network will get better for everybody as they start throttling those who were using the most data.
Unless they spread the billing dates across the entire calendar, this could lead to interesting data throughput cycles. Obviously, it would be best if they spread out the throttling.
What they probably should do is throttle the top 5% of instantaneous use when the network gets above 90% utilization. Most of the time they wouldn’t have to slow things down, only when usage was peaking, and then they’d throttle whoever was really using it at that moment.