Why do they alternate allergy shots in each arm ?

No particular reason. Some offices alternate arms when only one shot is given each visit. Some switch arms when two or more injections are given per visit. At our office we do not alternate arms or we defer to patient preference. There is no data that i know of suggesting that alternating arms for immunotherapy either improves results or decreases adverse reactions. I'd like to hear our other experts' opinions.
Decrease Scarring. Is the only reason i could think of. At our clinic we also leave this decision up to the patient.