question about old hardwood floors

in my 1920s home we have hardwood thoughout.. the old floorboard subfloor runs the same direction as the hardwood flooring. at each joint in the subfloor i have a gap of about 2 to 4mm in about 50percent of the flooring (mostly in the high traffic areas). is this typical in older homes? i would imagne that if the boards ran perpendicular to the subflooring this wouldn't have happened.. but im guessing whoever built the place is no longer.. and probably not taking questions..

i know why this is happening.. but is there anything i can do to repair this without tearing out all the flooring and replacing the subfloor?

also.. if i have a loose or extra creaky board.. is there any method of securing it down? without major work...

thanks