Aww, C'mon 500grains, that's deliberately misleading.

The .500 Nitro SOFT POINT driven at a stupid 2500 fps (way too fast!) blew the first 4 inches of wet-stack completely off the rack, and still created a 4" wound channel more than a foot deep. The second one at 'proper' velocity made over 2 feet. The .500 solids penetrated 48 and 55 inches. Where's the surprise here?

The .25-20 solids made 14-inches at the slow velocity, and 20-inches at the slightly higher velocity. Entirely consistent with what we would expect from a low-to-moderate velocity solid.

Linebaugh Penetration Test Results

I do agree, though, that wet phonebook penetration should not be used to predict actual penetration on game, but in fact it is probably no worse than any other medium. As I have stated many times it is the perfect way for the layman to COMPARE the performance of one bullet against another, and to check the expansion characteristics of a particular soft-point. Its big advantage?: it is cheap and available, compared with ballistic gelatin (the industry standard).