Workouts versus tempos: Max heartrate difference?

my guess is that you're not running long enough at those other intensities to get your heart rate comparatively high enough.

You're comparing ~60-80s efforts with a 1200s effort

I've been doing a Jack Daniels 5k/10k plan and have an HRM for measuring hear rate. Workouts from this past week:

400s at mile pace: max HR hit per rep started at 162bpm then was 168bpm on my last rep (12 x 400m session)

"Hard" time intervals at just quicker than 5k pace (distance ranged ~600m to 1.1km long): first interval (4 minutes) of about 1.1km hit 166bpm, last interval was a shorter 600m (2 minutes) which hit 174bpm.

Both workouts on the same track with similar weather. So the longer 5k-ish intervals definitely had a higher hear rate for me. The 400s seemed to be too short for HR to spike higher.