Blood pressure is reported against named categories, yet the underlying measurement is continuous. The boundaries are chosen rather than discovered, and knowing how they are chosen explains why they differ between guidelines.
The measurement is two numbers
The higher figure records pressure in the arteries while the heart contracts, and the lower figure records the pressure remaining between beats.
Both are reported in millimetres of mercury, a unit inherited from the mercury columns originally used to take the measurement.
The two numbers carry different information, and which one dominates risk estimation varies with age, which is one reason categories are defined using both.
Risk rises continuously, not in steps
Population studies show cardiovascular risk increasing steadily across the range of blood pressure, without a natural break where risk suddenly begins.
Categories are therefore imposed on a smooth curve to make the information usable, since clinicians and patients need a decision point rather than a gradient.
Someone just below a boundary and someone just above it have almost identical risk, even though they are described differently.
Boundaries move when the evidence base changes
Guidelines lower or raise thresholds when trial evidence suggests that intervening at a different level changes outcomes.
Because a threshold change reclassifies a large group at once, such revisions are debated extensively before adoption, and different bodies reach different conclusions.
This is why a reading described one way under one country's guidance may be described differently under another's without either being incorrect.
Measurement conditions affect the number
Readings vary with time of day, recent activity, posture, cuff size and the setting in which the measurement is taken.
Pressure measured in a clinical setting is often higher than the same person's readings at home, a well-documented effect that guidelines account for explicitly.
For this reason categories are generally applied to an average of repeated measurements rather than to a single reading.
The category is a starting point, not a conclusion
Guidelines use blood pressure alongside age, family history, other conditions and additional measurements to estimate overall risk.
Two people in the same category can therefore receive very different assessments, because the category is one input into a broader calculation.
Interpreting a reading and deciding what follows from it is work for a qualified clinician with access to the full picture, not something a category alone determines.