Minor versions go up according to a certain pattern based on how the change mgmt is handled for the kernel, but major releases are marked when the kernel team "feels it's the right time".

From lwn: https://lwn.net/Articles/871989/

To kernel developers, version numbers continued to have no meaning other than as a measure of where things were on the timeline, he said; users were encouraged to run the latest version. But if you tell customers they need to move from 3.20 to 4.0, they will consider it a multi-year effort that requires planning and paperwork; "so they will only do it when they have time, which is never". Going from 3.20 to 4.0 seems like a big jump, and 4.0 to 4.2 seems much smaller, even though the latter brings roughly double the number of changes.

The version numbers are not really meaningless, because anything higher than .20 is life-threatening to Torvalds. But technically and functionally, it doesn't matter.