safetypee, thanks for the info. The table in my post is just my own formatting of the original tables from the latest version of the -100 and -300 FCOM Vol 3 Pt 4 (Flight Deck Handbook). 502 engines for the -300, not 507. I was not flying the type prior to the major FCOM rewrite, I understand the old "MomVol's" were far less user friendly.
The graphs from the flight manual show straight wind lines which would suggest to me that there is no additional factor for the 15 knot column. We also have data for 20 knots tailwind, that does have an additional 10% factor.
The FCOM tables and flight manual graphs include a 67% safety factor so any variations in the original flight test data would be magnified by a similar amount.
Brakes and tyres are carbon and high pressure for both types.
From what you say about how testing is done, I'm thinking it's most likely just a result of minor variations in flight test results.