I would be a wealthy man if I had a $ for very time I’ve been asked that question. Almost every time I take my '65 out, somebody asks the 2+2 question - I'm sure you can relate, right?
The answer nearly always elicits disappointment, because it is so simple. Here goes:

John DeLorean, Chief Engineer of Pontiac in the early ‘60’s, and later the head of the Pontiac Division, was fond of European sports cars, and would borrow names from them. From Ferrari DeLorean got the name GTO, an acronym for a grand touring car certified (homologated) to race in a certain class. When he wanted to market the Catalina as the “Big Brother” to the GTO, he chose the name 2+2, a designation used by Ferrari and Morgan (and later by the Mustang).
What does 2+2 stand for? The number of seats – a sports car with 2 seats in the front plus 2 seats in the rear. For the Catalina, Pontiac was playing a little loose with the truth, as it was a 5-seater, but “2+3” just wouldn’t sound right.
Most people who know cars but don’t know the Pontiac 2+2 assume incorrectly that – like the Oldsmobile 442 - the numerals refer to performance options (in the case of the Olds 442, 4-barrel carburetor, 4-speed, dual exhaust – even though they also fudged the facts by offering a 3x2-barrel option and an automatic transmission).
Remember, on the 1964 Catalina, the 1967-70 Canadian 2+2's and the 1986 Grand Prix 2+2, it was only an appearance option. Only on the 1965-67 models were the performance options a standard part of the package.