The +53mm offset is the correct stock offset for the STi and many other Subaru models. This is the "ideal" offset for any wheel you put on the vehicle.
The Tire Rack offers most if not all of their wheels for Subaru applications in +48mm offset. While it is not exact, it is close enough offering the best compromise in a variety of wheel widths.
Some believe, rather strongly, that offset changes as wheel width changes and that is the reason for the range list. IMHO, the list is not really correct.
In my experience over the years, keeping offsets as close to stock as possible yields the best results.
Bottom line, the +48mm offset wheels offered by Tire Rack and others will work well on your car. The STi BBS wheels with +53 will be even better.