This is entirely speculation on your part. It could just be that they are keeping their sales prices proprietary because some books sell for more as well. It's a two way street.
That only makes sense if the hammer prices were only available to those who bid. While the duration for finding them isn't particularly long (depending on the auction house, some are very long), the final prices are public knowledge.
Maybe they have other reasons, but GPA has gone on record on the boards more than once stating that the reason some of these auction houses aren't in the tool is because they wanted to pick and choose which data to report, which GPA was not supportive of.
:shrug:
CLink does consider their sales price proprietary, which is why GPA can't just harvest them all for their site.
Just as there is a benefit to knowing what record low sales were, there is also benefit to a seller to know what record high sales were.