You also have an anaerobic component on short high intensity intervals. HR does not capture that effort. Perceived effort does. You feel the short term energy systems exhaust themselves at 90 seconds and then again as you start building lactate.My question isn't if my estimate MAXHR is accurate, but is why would the same HR zones in a shorter higher intensity session feel less efficient then the same ones at a longer lower intensity session. That seems feels counter intuitive.
HR also lags badly. If you try a max 1K without warmup you CANNOT hit max HR, but will be dead from a perceived effort level.
Longer, slower intervals let you hit max HR without the lag so perceived effort at the same HR is less, and without getting as high into your lactate threshold.
You can also see this effect if you look at your HR curve for intervals. My HR climbs with each interval -- starts a little higher and ends a little higher on each interval. My 2nd or 3rd intervals (at constant output) tend to be the lowest perceived efforts. YMMV.