I used to be the author of the Austin-Healey prices in the Sports Car Market "Pocket Price Guide" (and also the Triumph prices). However, a year or two ago they changed to using sales results. This has the advantage of saving money - they're no longer paying consultants - and it's inarguable as it is based on actual sales.
It may be less indicative of the overall market as it is based on a market slice that does not include private sales, but for auction-goers it is good data.
So to answer your question, the "reason" for the price difference is the marketplace, based on the cars that were offered and purchased. Opinions may differ on whether or not those results are indicative of the overall market, but that's what the slice of market that they can see has shown. It results on some strange anomalies such as a BN6 listed as worth more than a BJ8 (which would never have happened when consultants were authoring it), but think of it more as a market data snapshot rather than an overall, long-term "worth" of the cars.