Noticed that too. We just upgraded all our servers from M3 to M5 instances, but all in the us-east-1 region, so looks like we won't save a lot by purchasing more reserved instances.

Noticed a huge 20% reduction in some other areas though...

The us-east-1 region is widely considered as the "default" region, and so it's where customers put resources when they don't have any particular reason to put it somewhere else. For that same reason, it's also where less-sophisticated customers are likely to run unreplicated workloads. It's also the oldest, and biggest, and home to the most different instance types and rack architectures. It makes life hard when your largest, oldest, weirdest, and most important region are all the same region, so AWS has been trying to nudge customers off of old instance types and out of us-east-1 with pricing changes for a long time.