first, you are clearly missing the basics of what "offset" is. Offset is where the hub mounts with relation to the wheel. in that a 7" wide wheel with a 0 offset with have 3.5" in front and behind the wheel. 0 being the center point of the wheel.
a 7" wide wheel with a +25mm( ~1") offset has 4.5" of backspacing and 2.5" of front spacing
a -25mm offset on a 7" wide wheel will get you 4.5" of front spacing, and 2.5" of rear.
on a 8" Wide wheel 4" on either side of the hub, will give you a 0 offset.
it is a calculation to dictate how much back spacing and front spacing you have so that you can figure out clearance between the suspension and also the fender well.
38mm offset only tells me 1/2 of the story because I don't know how wide is.
this a decent chart for reference. my wheels are far different than "OEM SPEC", but with a little measuring you can fit a whole lot more than the lame wheels mazda puts on our cars.
if you can be more specific with you question, we can be a whole lot more help.... and won't have to ramble nearly as much next time