It sounds like they're going with what's most likely to actually work, so as not to wast time waiting for cultures, which can always be a bit hit and miss.
I note that non-medics tend to think of second-line antibiotics as 'stronger' whereas it's better to think in terms of 'appropriate to the organism', traded off against side-effects, ease of use, and risk of encouraging resistance. You don't go around routinely blatting things with vancomycin because you can't just give the patient some pills four times a day, and you want it in reserve for the MRSA and C. difficile. But equally you don't want to let a bone infection take hold while the microbiologists do it with culture and sensitivity. Medicine is as much art as SCIENCE.
