Multisite w/ domain mapping

Hi, I’ve noticed that all files, also those for subsites in a multisite network installation, are stored in uploads/omgf/… Is this by design or is this happening because the sub-sites in my network are using dedicated domain names instead of subdomains or subdirectory paths?

So far, this has not caused conflicts on my network because none of the sub-sites use the same theme, but I would guess this might be different when two sub-sites use the same theme (or plugin with google fonts) but different settings.