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?
Could be that book 7 had to be printed on lighter-weight stock if the page count would have exceeded the capacity for the hard-cover binding. (They may not have anticipated that a later book would get that big when they chose the stock at the start of the series.)