It's a confidence thing. Janssen can now choose between a shirt number whose last two owners were flops, or one whose last two were a success. Does it really matter, probably not, at the end of the day it's just a number, but I wouldn't rule out him picking the 18 ahead of the 9.
I don't consider Pav to have been a flop.
Also, I really don't think Janssen cares about the last two players to wear the number at Spurs. That's something that fans worry about, not players.