Signs in prayer rooms do tend to be of the makeshift variety, IME.
As to your question, because of the nature of Muslim prayer - bending down, prostrating, etc, men and women often prefer to pray separately, so the prayer space may be divided in two and each side may be accessed by a different entrance.
People may pray in a congregation, but often prayer rooms are just used for people to pray by themselves, so they just nip in and out.
Or there may be separate washing facilities for men and women to use prior to prayer. Separate facilities are preferred for this, as you need to wipe over your hair, so it's better to be somewhere where you can take your hijab off.