Stock wheel offset compared to aftermaket?

Ok so I am looking at a set of titan rims they are 18x8 and it appears as the picture is hard to see but it shows the offset on the rim stamping as 25mm now I know these tuck with a little modification. However how come with a 25mm stock wheel it will work but for an aftermarket wheel we have to go way high? I know i saw at one time a +31 will also tuck so could I get a wheel that is +25 and it will tuck? I have a frontier so I know my fenders are closer then an s10..just trying to find out I guess..As I have seen a ton of 6 lug chevy wheels for sale around here cheap but I have only been looking for the very high offset so hopefully someone can answer this question.