This is the bottom line.
F&A surgery is done by many specialties: pod surg, gen ortho, F&A ortho, trauma ortho, vascular, plastics, derm, and even gen surg sometimes.
A gen ortho might have had a high volume F&A trained guy at his program - or maybe even just a gen ortho attending who happened to do a lot of F&A cases. In a 5yr gen ortho program, they get a short F&A dedicated rotation, but they also run across plenty of F&A stuff on their gen ortho, trauma, sports, etc rotation blocks. If they decide after residency to prep on the F&A anatomy, they obviously knows the principles of bone/joint and musculoskeletal surg... as well as AO (internal fixation, plates & screws). If a gen ortho then feels comfortable and efficient at F&A and will prep for the tough cases, then he can go for it. Chances are usually that most gen orthos focus more on higher pay procedures (total joints, long bone fx, etc) and stuff they're more efficent at, but if a gen ortho guy gets good income, satisfaction, and patient results on F&A, then by all means.
Vasc surg obviously does the amps and revascs in the lower extremity. Derm and plastics do the soft tissue stuff. In the end, it's about interest and comfort level. If a surgeon (of any specialty) is doing procedures they're marginally qualified for or have little to no residency/practice volume with, then they will typically be short lived due to one reason or another... can't get privileges, malpractice, poor results and low efficiency leading to them abandoning those types of cases due to frustration, etc. Everything has a way of working itself out.
We're back to the 4 pillars of good surg:
1 Personal interest and motivation
2 Education, training, prep, and exp
3 Natural motor ability and hand skills
4 Balls (err, confidence level)