Both Princess and her predecessor Madame have nested on the ledge so can't entirely blame Princess for poor decision-making, but certainly, she has nested there more often than Madame ever did. And Princess was hatched in a nestbox - not sure about Madame, it was a long time ago and the records are sketchier but I suspect she did as well.
Can't even blame the men in their lives because Trey was Madame's mate and Princess' mate - T-Rex was only Madame's mate and he started the ball rolling back the year Trey hatched. Ivy was from the box and Smiley came out of a nestbox so they don't have memories of the box, they just make scrapes wherever they find a suitable spot. Having said all that, right now it is Princess that is encouraging Smiley to make scrapes on the ledge - when he arrives he is stuck to the nestbox like glue and when she doesn't show interest, then he follows her and will make scrapes on the nestledge in order to please her. He did this last year and this year - this year she chose the ledge. With Ivy, in the 4 years they were together, she used the nestbox for the first three years, ledge (Tenzing & Hillary) for the last.
Why she chooses which may have something to do with their scrape making skills perhaps? I don't know - certainly Trey made lovely scrapes and Ivy was a digger, his scrapes looked like craters in the nestbox, she used to smooth them out a bit once she started incubating. Smiley's scrapes are good but they aren't as deep or perhaps as big as Ivy's and Trey's were. Perhaps it is a combination of the scrape itself and the area it is in, perhaps the ledge is bigger and his scrapes are more symmetrical there? Maybe the years Trey's and Ivy's scrapes on the ledge were chosen the ones in the nestbox were too close to the back wall and therefore weren't symmetrical or or or or? Couldn't say, it really is up to the ladies in their lives I'm afraid. I should say that she is far from the only female that alternates between nest locations at a particular site or within a territory, at least she sticks to the same building, makes it so much easier when it comes to cameras!
Or perhaps she just likes to keep us guessing.