how does mke2fs decide automatic check mount count?

Whenever you create an ext3 or ext4 filesystem with mkfs or mke2fs, a message is printed saying "This filesystem will be automatically checked every X mounts or Y days, whichever comes first".

However, the number seems to vary depending on the filesystem. How is this value calculated? I used the source of e2fsprogs-1.42.8 and the cscope source code tool to find this out.