Maybe this will clarify
Peppermint has always released annually, and early on this worked out fine because we originally only (officially) supported one release until the next .. this worked because no matter which upstream version we were based on also had a minimum of a year of upstream support.
Then Ubuntu switched to 9 months support for non-LTS releases.
So now (like Mint) we ONLY base on LTS , but update the kernel via the HWE:
https://wiki.ubuntu.com/Kernel/RollingLTSEnablementStackSo....
Peppermint 7 is based on 16.04 LTS
Peppermint 8 will (likely) be based on 16.04 LTS but with the kernel from the HWE (whatever that is at the time)
Peppermint 9 will (likely) be based on 16.04 LTS but with the even later kernel (on the iso image) from the HWE
Peppermint 10 will (likely) be based on 18.04 LTS
It also helps that Ubuntu have now made the HWE stack rolling, so Peppermint 8 and 9 will get rolling HWE updates right up until 18.04/Peppermint 10
So Peppermint (which release annually) now only switches base completely when a new upstream LTS is released, but the interim Peppermint versions still get newer kernels and graphics stacks via the HWE
Did that make sense ?