Patrick Gainer established his reciprocity formula based on data from this article:

Ilford hasn't changed it's reciprocity data for HP-5 for decades, but the film has changed. Here's a quote from Bond's article concerning HP5+.

Ilford curve outdated, too?
In publications dated September 2002 (sent to me in January 2003), Ilford still had a single old curve for correcting reciprocity departure for HP-5+ and 100 Delta. I found the behavior of these films to be quite different from the curve, and also different from one another.
You'd likely do much better with Bond's recommendations for reciprocity adjustment for HP5+ than with the data sheet from Ilford.