Most of the 427 street cars (not 427S/Cs) had 428s in them. Most say the reasons were cost and availability. The 428s were cheaper than the 427 side oilers.
Of course, Ol' Snake
Oil Shel left the 427 badges on cars with 428s. The story is that Shelby didn't mention the 428s unless he was pressured to answer specific questions. In other words, most 427 buyers didn't know they bought a 428.