[ohhawk]

"As it was, the leftover "396" identification on the fenders was incorrect anyway since the '70 Super Sport's standard Mk IV big-block in truth displaced the 402ci thanks to a minimal overbore first done very late in the 1969 model run. Not wanting to confuse a good thing, Chevy officials continued to advertise the '70 SS Chevelle as a 396, and all mentions of standard power listed the "396 Turbo-Jet", as did the air cleaner decal under an SS's hood. Scrambling the picture even more was the new 330hp LS3 402ci big-block, which Chevrolet labeled a 400. Not to be confused with the equally new, correctly identified 400ci small-block--a siamesed cylinder evolution of the famed Chevy V-8 that first appeared beneath the "Hot One's" hood in 1955--the LS3 Turbo-Jet 400 was available on all V-8 Chevelle models except SS 396s, the first time a big-block was offered by Chevrolet in an A-body without SS equipment."

....quote from Chevelle 1964-1972 Muscle Car History book

Sounds to me like the short answer to the original poster's question is MARKETING.