That was sharp of you to notice that... especially with the doors closed.
You're 100% right.
The back doors have frames and the front doors don't. That began in 1971. The 1970 and earlier had pillars for all four doors. The rear doors were cut into the roof to aid in entry and exit.
I did ask one of my Cadillac friends about that. He guessed that maybe Cadillac eventually figured that it wasn't necessary for the chauffeuer to have ease of entry/exit.
Here is a pic showing the doors open and both windows about 5" down.