It's your blood pressure pal here.
Just to give you a quick bp tutorial! Your top number is the pressure of the blood in your arteries when you heart is in the middle of a beat, and the bottom number is the pressure when your heart is at rest. The top number changes frequently depending on the load (strain) on your heart. So if you are sleeping, the top number will be lower because your heart is not working very hard when you are asleep. If you just ran up a flight of stairs, your top number will normally be higher because your heart is beating harder, faster and working more.
The bottom number is particularly significant because it shows the amount of strain on your blood vessels (and organs) when your heart is in between beats. It can also increase under a high workload (exercise or stress) but it won't go up as high as the top number which is when your heart is in the middle of a beat. The bottom number gives a clearer picture of how well your blood vessels recover between beats. There are a lot of chemicals in your body that regulate this, it's all kinda complicated and I'd have to look it up to tell you exactly what happens... the kidneys play a big part in it along with other hormones in your body.
Does that help?
As far as I know, it's not unusual for the fetal heart rate to slow up a little at the end of pregnancy, And, it sounds like the baby was sleeping, since the buzzer made him suddenly move and increased his heart rate. That sounds like a totally normal reaction and totally normal NST. If baby is sleeping, 120 is fine. I occasionally get that when mine is asleep.
Can you get a BP cuff and take your own at home? I take mine every day (only because of my history, I'd been on meds almost 2 months at this point the last time) and bring it in - so if it's a little high from me running and rushing to get there, I can show them that it was normal all week and normal at home before I left. Even if he hasn't asked you to do that, it might be a good idea.
I agree with your body only growing what will fit through your pelvis. Sometimes babies appear to not fit, but perhaps the labor was not managed well and the baby was in a poor position to descend. I can't imagine that nature makes a bodily function that fails 30% of the time... that just makes no sense to me! Especially one as important as continuing the next generation of the species - that's pretty important!!
Hope that helps a little!