When I test people with such high thresholds I’m not surprised when they get a response on one test and then not on the next, although the change on the Right would be more surprising than the left. I’d wait to see if it’s the same on another hearing test, maybe even two before attributing it to a permanent decrease. It also depends on the transducer used during testing. For example, if I test using headphones and not insert earphones I can get 10dB more volume…so I can go up to 120dB instead of only 110dB, when I use headphones. So if your pro used different transducers that could explain part of it.
As to why it would effect the lower frequencies…there are some syndromes that target the low frequencies, like Meniere’s, before High frequencies.