There used to be a way to abuse location edits to force this situation. This is why the current distance you can suggest for a location edit is so limited. If the L14 is ever recalculated for some reason, the extra gyms should go away.
Thanks IAm moving this to general discussion as it deserves more visibility.
It may not be the case here, but “sponsored” gyms can be added in excess of the three per grid limit. So if any of these are “sponsored”, it could be that the grid already had three before that one was added.
However “sponsored” stops and gyms don’t automatically appear on the Wayfarer map (you can however manually add them), this is why I am guessing this doesn’t apply here.
Given that cluster of three is so close to the grid edge, my guess here is one of those was location edited over the grid line.
Looks to me like help chat may have edited the location of a gym from the cell to the south, which is also underserved now (yellow)