Question about HIIT

Question for you all you are more knowledgeable in this area...

I go to the gym at work a couple times a week and do some resistance training (no heavy lifting as yet) on the machines (that's what's available) and then 40 minutes of cardio on an AMT machine. I'm a geek and love to watch the numbers; I wear a HR monitor. Usually I will do a steady state cardio routine one time and attempt some HIIT the alternate time.

Here's what I'm wondering about: when I do steady state cardio, I can go hard. Example: last Wednesday my average HR over 40 minutes was 163 and I was cruising (aerobic zone). I sprinted for a minute at the end and got my HR up to 187 (max ever). (I'm 43 yo female, so that's pretty high for a max HR.)

On the days when I try HIIT... The intervals on the machine are set to 2 min on and 2 min off (that's the shortest available). On the intervals I sprint as hard as I can, and on the rest cycles, I go slow and bring my HR down to about 127-130. On the sprints, I go anaerobic, and can feel the lactic acid build-up. I drink twice the water during once of these sessions and sweat a lot more... But on the sprints, my HR never goes as high as on a steady state cardio work out. E.g. last Friday I got a max HR over the whole work-out of 167. I was anaerobic with a HR of about 155 on the sprints.

I'm assuming this kind of changing around of training is a good thing, and clearly I'm working things differently with these different approaches. But can anyone explain what's going on? I thought with HIIT you were supposed to try to get your HR up as high as possible to spend as much time as close to HR max and ultimately increase fitness and your VO2max.... But my HR never, never is as high on an HIIT session as it is on any steady state cardio workout. I'm OK with that, I would just like to understand what's going on and/or if I should be doing something different to achieve the usual objectives of HIIT? Or is working the anaeorbic pathways like this OK, even if my HR is not as high as it could be.

(And any links to science behind this welcome too, if anyone has good references.)

Thanks in advance!