Any explanations as to why i'm running a similar HR at very different paces?

This has been a recurring thing, not just a one time fluke.
My long, slow runs lately have been around a 10:30 pace, and within a mile or two my HR is up to z2(160ish, give or take. My field test, yes I know how to do it, has my z2 between 155-167).
My mid week runs, are also supposed to be mostly z2, with some z4 intervals.

What I've seen happening is that my HR has been going up fast on my long workouts. Like I said above, i'm in the upper 150's within the first mile.
I've been dropping the z4 work from my midweek runs because whenever I hit that intensity i start feeling an old calf injury. It doesn't hurt yet, but the spot where I felt it becomes noticable. On my run yesterday my goal was just to sustain a z2 run for the full hour. I paced under 9 min miles, and I didn't hit the 160's until mile 5. And I kept getting faster as the run went on. I started around a 9:15, then gradually wound up at 8:50 by mile 4 before slowing a bit. But, still, significantly faster than my long runs. I've also been doing a 9 min run/1 min walk for all my runs. I just find that I'm faster overall by doing this, and it's the only way I keep my HR in z2.

I run at the same place every time, so the terrain is identical. I run in the morning on my long runs, so it's cooler, although the humidity is higher. My midweek runs are at 5 pm, so it's still hot, but less humid.

Could humidity play that much of a factor that I can run 1.5 min/mile faster at the same effort?