IPC values are rarely logical.
We already know that Belgium, Holland and Serbia are each worth 2. But while the Low Countries were heavily industrialized with large colonial interests, Serbia was a backwoods mountain Kingdom with little or no industry and an army little better equipped and organized than partisans (though relatively large in numbers).
GSWA seems to have an infantry and artillery while the other 3 colonies just have the one infantry defending. If we assume that Germany cannot build units in Africa it doesn’t give them much scope to do anything bar sit tight and hope to survive.
It’s a little awkward to rate these colonies; SWA had by far the smallest population, but it had the highest population of Germans (there were mineral deposits in the desert; the other colonies were largely jungle.)
Again it would be logical that players can recruit the odd infantry unit in African colonies, but this doesn’t look like being the case.