Okay, I'll give up the "secret". :-)
Gm wanted to introduce the new "convertible-look" roof on the upcoming 1962 model hardtops!
They had a little problem though.... they had a large amount of roof pieces already manufactured and warehoused, that were for the hardtop style they had been selling though '61.
What do you do with a large inventory of parts that you suddenly don't need... but have a lot of $$$ invested in??? If they threw them out, managment or investors would probably be very upset!
Eureeka! You put them on the less expensive models, and use the new roof only on the more expensive Impala! You might even attract buyers who would pay more to get the new styling! :-)
....so as I suggested, it was a decision based on practicality. If they hadn't worked so far ahead, the BelAirs would have been built with the same new roofline as the Impala.
Blue.... Ya gotta question yet?