"How can it be too easy to capture fields on one map and not the other when fields have the same layouts on all maps?"
Field layouts are the same, but Mindanao is much less symmetric than the other maps. Quick resets are usually the result of 2v1 gang bangs. One the Mindanao map, the NW country will never be ganged, because of the narrow access route.
It might be interesting to turn sector counters off for one rotation of the Mindanao map, and see if attacks on the NW would be more common. (It is pointless to bomb HQ to disable dar prior to an attack, since resupply is so easy.)