The general behavior results from polygons at one level not being quite closed in regards to the id variable.
Here's a brief example:
data example;
input id x y id2;
datalines;
1 10 10 1
1 10 15 1
1 15 15 1
1 15 10 1
1 16 27 2
1 25 27 2
1 25 12 2
1 16 12 2
;
run;
proc gmap map=example data=example;
id id;
choro id;
run;
quit;
proc gmap map=example data=example;
id id2;
choro id2;
run;
quit;
Note that for ID the boundary continues (connects to) the area identified with id2 when using ID.
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