1200mm between wall studs..?

I am quite new to all of this so bare with me. I am trying to put up some shelves but it seems the distance between the studs in my wall is 1200mm. From searching online it seems that 400mm or 600mm is the standard. Can anyone tell me why this may be the case?

I've checked other walls in my property and they all seem to have the same distance of separation.