I'd agree with BOAC's comment that even a small RPM difference can affect pitch authority more than you might expect.
Also, given that, as BOAC says, it seems to be pitch authority loss that is being used as the 'stall' determinant, are the two airframes identically rigged? A small difference in elevator up stop position would have a significant effect - even one or two degrees. As would any slight variation in tail incidence. The older the airframe, the more likely that there are small variations. ??
A thought on trim position, as previously mentioned. On one light aircraft I used to regularly fly, the nose up pitch limit was slightly, but noticeably, improved by appling full nose down trim. Of course, that is unlikely to be relevant in this case because the testing pilot would tend to trim nose up as he slowed towards the stall anyway. My working against the trim force is to ensure adequate pitch authority when landing in a taildragger with a rather light tail load.