It's a odd size but more for being too stingy - normally in these kinds of setups you get something like a /48 or /56 for routing which lets you still have some

It's a odd size but more for being too stingy - normally in these kinds of setups you get something like a /48 or /56 for routing which lets you still have some bits for routing and then you can hang normal /64 subnets at the end of your routes. Operating systems don't support non-64 prefixes very well and v6 wasn't designed to work like that.