I know that some, like Picadilly, post early so that people maybe buy the records on their site for the holiday season. Others, like NME and Rolling Stone, have a print issues coming out in December that will surely have their lists in them, and print takes a while to be publication ready, so they likely publish their list online once they’ve decided on it for their print releases. I’d bet sites like CoS and DiS want to get out ahead of the larger competition like Pitchfork and have their opinions heard before they’re drowned out. Similar to primary elections in the U.S., earlier contests get to set a tone or gain momentum for an album (or, in the metaphor, candidate), and so entitites that move earlier get to play a bigger role in influencing what consensus opinion develops.
Remember, most of these sites are looking to make $ by boosting their clicks and strengthening their influence, and the sanctity and accuracy of their lists comes 2nd to that goal. On the contrary, Pitchfork and Needle Drop are already the preeminent taste-makers with the largest influence, so you’ll see them be careful to wait on their lists and value some kind of objective accuracy with fewer asterisks caused by December releases in an effort to maintain their positions at the top of the mountain.
I’m not a music industry professional, this is just my educated guess on the topic.