Selective double tracking
Tell Me Why features double tracked vocals in the stereo version but only single tracked vocals in the mono.
"I think a lot of these [John's] songs like "Tell Me Why" may have been based in real experiences or affairs John was having, or arguments with Cynthia [John’s wife] or whatever, but it never occurred to us until later to put that slant on it all." - Paul
- Beach Boys (from Beach Boys Party!, 1965)