If you export 2 cubes named + colorised in skp format from Moi
one separated (explosed) and one not separated
Now result in Sketchup will be the same ;)
all cubes will be similar, and they will be not a group!

Now make the same but export as 3DS format from Moi!!!
Import it in Sketchup as 3ds format
result appears as a general group : explode it one time
You will have perfect result ;)
(except colors, no colors in sketchup from Moi till now) maybe V3 :)
And name are replaced by OBJ1, OBJ2...OBJ6, OBJ


you will obtain
first cube = 6 groups!
second cube = one group!

It's not so bad is not it? :)