You may be right as I didn't have the Kindle with me for a side-by-side comparison (my daughter said I had to leave it at home if I was going to the mall with her
).
I just spend so much time with my Kindle that I noticed a difference but it could've been the store lighting.
Another thought - Although the 3 font sizes on the 505 were fine for me (as opposed to 5 for the Kindle), I noticed tighter line spacing with the Sony, which was closer to that of a paper book, and wondered if that would make it harder to read for long periods as well.