AWS very often charges across multiple axes. I tried to model out our Cloudfront charges and they charge there for 3-4 different factors, each of which varies in pricing by region.

I think the idea is that by charging precisely where they incur costs, they can be much more reactive to different usage patterns, and therefore be more competitively priced overall.

Although it certainly does create lock-in due to not being able to figure out your billing and accurately model alternatives.