Early-1990s protocols were less thorough.

The collagen extraction and pretreatment methods available and routinely used in 1991 were not as effective at removing potential contaminants.

This is not a criticism of the specific laboratory that ran the Buhl date — the University of Zurich facility was a reputable and technically competent laboratory.

It is simply a recognition that the field has advanced, and that a date run in 1991 using 1991 methods would today be subjected to greater scrutiny before being accepted as reliable.

A sample with suboptimal pretreatment might have incorporated some contaminant carbon that shifted the date — in either direction, though the specific direction would depend on the nature of the contamination.