The pages feel thinner and you can see the text on the other side more as well. Book 6 is a 10th printing and book 7 is a 5th printing. Doesn’t say years, so I wonder if they switched to thinner or back to thicker?
I did notice some of the middle books had a shocking amount of ink smudging and that multiple times while reading Parade of Horribles I found myself thinking I must have two pages stuck together because the pages were thicker than the other books