I've found the exact same thing.

If you were professionally racing that 2:19 difference could determine whether you continue to the next stage or not. On a commute...what's 2:19?

I do whatever my legs tell me. Some mornings I set out and the legs just want to spin easy, or they're feeling a little dull. Other mornings I intend to spin easy, and start out that way, but find myself sprinting for all I'm worth by the end.

Just enjoy the ride, however it works for you. +/- 2 minutes will work itself out later.