Hi FatDad (love your username LOL)!
there are a few reasons that could cause the changes you’ve seen:
1. you are running more efficiently and run faster at the same power output
2. you lost weight and are therefore faster at the same power output
Your LTHR should not change much, but you should be able to run faster at the same HR as your fitness improves.
I would recommend to check your Stryd or whatever running PM you are using, to see if the weight was updated or changed. If you are using an Apple device, Stryd automatically updates the weight by accessing your weight from the Health app.
I hope that helps.
Winoria