First thing that comes to mind is that the glass in the cheaper readers might be lower quality than your higher priced 1.5's. The readers that I use for books are $175 glasses, but I have 10 pairs of Dollar Store glasses to use for various other tasks.
I think readers are very sensitive to distance. I use different strengths for using the computer, reading books, and for working on gimbals close-up. If you are using a small screened device to fly, that might be the reason for needing the stronger readers, as well as the cheaper glass quality.