The link that Streets put up tells the story.

But this "misunderstanding" gives a hint as to why those of us who lived through the '60's are so nostalgic about some of these cars. The ability to have a car that looked like a granny mobile on the outside, but had the heart of a lion (or rat, or mouse, or ELEPHANT). We called them "sleepers". And this wide variety of factory built performance cars knocked the air out of the traditional hot rod "hobby" for almost a decade. Most don't remember, but there were any number of magazine editorials at the time wondering if interest in the old hot rods would die because of the "factory hot rods". Seems silly now!!