This is a bit of a guess, but I reckon the printer had 2 different batches of paper on the go, of different qualities. Look carefully next to the spine and you'll see it's bound in 9 signatures, 2B, 2W, 2B, 1W, 1B, 1W where B=brown and W=white. It's likely that the same effect will be visible in other copies from the same print run.
Each signature is printed and stitched, then assembled into the cover. The brown signatures were presumably printed on more acidic paper. As the acid can migrate from one page to the next, the sheet or two of white nearest a brown page to be a little yellowed. The effect is probably greater near the edges as ambient sources of acid also contribute. The brown pages/regions should be noticeably more brittle as well.
That looks like the 1961 (or '62 depending on source) cover; books I have of similar age show varying degrees of yellowing (e.g. 50s textbooks look good as new, many 70s novels are browned and have to be handled with care), so at roughly the same period a variety of papers were used.