The NBT for each chunk includes a long integer variable called "InhabitedTime" that is supposed to be incremented whenever the chunk is loaded with a player in the vicinity. However, this doesn't always get updated as expected. For some chunks, the variable increases more slowly than expected.

To test: create a new world in creative mode and simply do nothing for 20 to 30 minutes. Then save the world and inspect "InhabitedTime" for each chunk.

The screenshots were obtained by mapping the inhabited time for each chunk to a heat map with lighter values corresponding to higher inhabited times. Two worlds were created with the same seed (seed was 1).

Tested in the 18w11a snapshot but the problem is known to exist in older versions.