I don't remember Paulie ever being underboss. The underboss spot seemed to have been vacant for a long time. Silvio was Tony's number two and I know he was always seen as the consigliere. Then Bobby started making some noise. He seemed to be doing well financially, he was tough and came from good stock. Not to mention he was also Tony's brother-in-law. With all the morons working for Tony, Bobby was probably the only guy he could really count on. I don't remember ever hearing him being called underboss on the show, but he was definitely the number three. People tend to put a lot of stock into titles. In reality, this isn't exact science and it didn't seem to be on that show either.