Building slots on the arch

It appears that some people have extra building slots on the arch islands, particularly the island bought with gems on the right bottom corner. EvilJ and ghannima have the option to put a storehouse between the titanium and gold mines on their islands but I notice that other people do not have the option to build there at all. What is the explanation?
Thank you in advance :)

A couple of months ago, if not already a year, the map structure something something got changed by 1 pace or something something, which resulted in many of those "extra spots" to now being gone. I think that also fixed the issue of being able to overlap buildings in certain spots like on marble/iron/gold deposits and other bugs. So all in all, the earlier you have played and made use of certain bugs (removing shrubs, overlap buildings) the more spots you have. The archipelago wasn't a bug in and by itself, but got corrected at a later stage.