It is actually a completely baffling question, Roger. I lived there for forty years and I don't know the answer. I can't even recall ever wondering about it, or being in a conversation where the topic came up. Weird!
I quite like Perts though.
I did try to research the answer on the web, but have come up with nowt so far. However, I was delighted to find a photograph on Wikipedia's Perth (Scotland) page of my childhood home, where I grew up from the age of 4 to 21. It's the first door on the left of the picture, and my bedroom was behind the window top left (below the roof).
It was a fabulous place to grow up, looking out onto a huge park (that's the entrance at the forefront of the picture) that had everything a kid could ever want, including a huge domed sports centre, sports pitches of every description, and a golf course. At the far end flows a beautiful stretch of the River Tay.
Needless to say, I took most of it for granted as a kid, but I've lived in enough places since those days to realise just how lucky I was.
Bet you wish you'd never asked now, Roger - was there a particular reason?