NEC-LIST: Multiple Patch Surface

I used SM multiple-patch cards to create a box connected by a wire to a ground plane.
NEC instructions tell us to locate a wire at the center of a single surface patch (SP),
and NEC will divide the patch into four smaller patches with the wire at their intersection, so I did the same with SM.
It produced a serious error in the wire current (almost a factor of 10 too high) while passing all the usual checks.
Moving the wire connection to the edge of a patch corrected the problem, although it's probably safer to put it at a corner.
Does anyone understand what the model thinks it's doing to produce such a high wire current?

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