Different markets use different materials. F/J studs aren't very common in BCS, but they dominate the DFW market. Similarly, BCS is still highly a southern yellow pine framing market (although that is changing), while DFW is predominantly SPF. In the Waco/temple/Killeen area it's almost all hem fir, which is about the cheapest grade of lumber you can possibly use. Custom builders around here would prefer Doug fir in most cases, but they come at a premium price compared to hem fir or pine studs.
As for the size difference, it is definitely possible. Some Mills run the studs through a planer after the finger-jointing process which ends up knocking 1/16-1/8 of an inch off of the studs. Minimal, but noticeable. Structurally though they are still perfectly fine. It's all about less twisting. Production builders love them because they spend less time returning or swapping material, which minimizes downtime on a job. You also don't have to worry about a framing crew that's not necessarily worried about quality putting twisted or warped studs in a wall. For the drywall guys it makes life easier 99% of the time.