The "section width" (225 235 245 255 etc) is the widest part of the tire when mounted & it is not the tread width.

The reason a BFG VS. Goodyear (225/70/15) will be different dimentionally is the mounted rim is not STANDARDIZED - the BFG may have used 6.0" & the Goodyear 7.0" or vice-versa.