That hasn't been my experience. Our off the shelf blinds fit nicely inside the windowframe. I barely ever see any blinds that don't. I think you may have been taken for a ride with your "investment" PP.
There is no chance you did your whole 1920 house with Home Depot shades and the shades fit all the windows. Some windows, OK. If your house really IS a 1920 house, then you will have to admit that you have at least 5, probably 12, different sized windows. And all 12 of those custom windows from 1920 handmade by some immigrant craftsman do not have corresponding SKUs at Lowes in 2013.
cheers,
Lowes seasonal worker