>I’ve never seen a discussion how ‘reactive’ one is physiologically

Then you have never looked for it. This is pretty basic stuff taught in the very first year of the medical school.

>when stressed rushing to a doctor’s office visit

Even has a name: white coat hypertension.

>I suspect blood pressures varies wildly through the day, and this variation depends on a person’s physiology.

Yes it absolutely does. Just like your heart rate varies throughout the day, your BP keeps changing as well. That's why we like to measure it over 24 hours before any diagnosis.