Yeah, I don't really get why they don't use the stable kernel releases, of which there are many, rather than rolling their own.
After all, they reinvented everything from DE to init system at least once in past.
(They also have their own containers, LXD. I actually really like that one, please keep working on that canonical)
But I assume as Ubuntu follows an April release schedule, it doesn't always match with an appropriate LTS kernel.