A trim can be accurate relative to the fixture and still be wrong relative to the cooled part. Thermoformed geometry continues to change with cooling, shrinkage, stress release, and fixture loading.
Incorrect datum selection, unstable part seating, hot trimming, roll-feed indexing error, tool wear, and part-to-part warpage all contribute. A flexible shell may be forced into the fixture differently on each cycle.
Diagnose the reference chain
Dimensions should be measured before fixture loading, while clamped, after trim, and after final conditioning. The formed feature used as a datum must itself be stable and repeatable. For inline systems, web index and part position should be checked independently.
The trim fixture should locate without distorting the part. Cooling and conditioning requirements must be defined before final tolerance is evaluated. Software offsets are useful only after the physical reference chain is stable.