I happen to run an 8-mile route frequently and my Coros watch pretty consistently gives me a calorie count in the 900’s when I run it at a faster pace (in less time) and over a 1000 when I run run it slower (for longer). I know it uses HR to estimate calories burned but a faster pace on average should also, all else being equal, not result in a lower HR, so it looks like its model pretty clearly is heavily influenced by exercise duration.
How can you burn fewer calories running the same distance faster? I don’t rely on calorie count for anything, but it just seems annoying to know that my watch is dumb.