When I Export A SKP file Without Explode, the Material Of These Two Sides Are Opposite After Export

when i try to export a skp file which just has a Group.
if export it directly,i got the material of these two sides is opposite.
but after i explode the group.
then i can got correct material for each side.
may i mistake something ?
someone know this? pls note me,ths!!!

Materials / colours should be applied only directly to the mesh / face (usually on layer 0), and not to a group. Possibly you have done both.