It is most likely because there is a limit of how close to the resource the Mill can be placed.
sn-food-dropsite-distance
The maximum number of tiles a computer player likes to walk to drop off its food. Must be >= 3.
I guess if it is greater than 3 then it doesn't like to drop a mill closer than that.
In the CPSB it has...
Appendix B - SN Parameter Defaults
(from the internal codes)
7sn-food-dropsite-distance
This I think is for the Xpac maybe it was 5 for the AOK.
This CPSB was linked in a post in a AOKH thread.
There is also this.
quote:
dropsite-min-distance <resource-type> <rel-op> <value>
This fact checks computer player’s minimum dropsite walking distance for a given resource type. Long walking distances indicate a need for a new dropsite.
It is not recommended to use this fact for building of first dropsites necessary for age advancement. If, at the beginning, the resources happen to be close enough to the Town Center, building of the first dropsites will be delayed, resulting in slower age progression.
Hope it helps.