Wearable systems report headline numbers such as load, strain or readiness that appear comparable across products. They are not, and the reason is that each is a formula rather than a measurement.
A derived metric is a decision, not an observation
Raw quantities such as acceleration and heart rate are physical measurements with defined units. A load score is a combination of those quantities weighted according to choices made by the vendor.
Those choices determine which movements count, how heavily each is weighted and how the result is scaled. Two vendors making different choices produce different numbers from identical raw data.
Neither number is wrong, because there is no external standard for what a load score should equal.
The formulas are usually not published
Vendors treat these calculations as intellectual property, so the exact weighting is often unavailable even to the clubs paying for the system. Documentation describes the concept rather than the arithmetic.
Without the formula, users cannot determine whether a change in output reflects a change in the player or a change in the software.
Firmware updates that alter the calculation therefore create discontinuities in a club's historical record that may not be announced clearly.
Scaling choices hide the underlying differences
Many systems normalise their output onto a convenient range, which makes the numbers easy to read and disguises how differently they were produced.
A score presented on a familiar scale invites comparison with any other score on the same scale, even where no relationship exists.
The presentation is doing the misleading rather than the calculation, and it is largely a design decision about user comfort.
Switching supplier resets the history
A club changing systems cannot simply continue its existing thresholds, because the new device's numbers occupy a different range with a different distribution.
Rebuilding individual baselines takes time, and during that period staff are working with less information than before the upgrade.
Parallel running of both systems is the standard remedy, though it doubles the equipment burden on players for the duration.
Raw data is the portable layer
Clubs that retain access to underlying positional and physiological data can recalculate their own metrics consistently regardless of supplier.
That capability requires analytical staff and storage that many organisations do not have, which is precisely why packaged scores are attractive in the first place.
The trade is between convenience now and continuity later, and clubs that have changed supplier once tend to negotiate hard for raw data access the second time.