I think it was probably a simple case of having the wrong frequency dialed in on the comm panel.
If you listen to 121.500 for about two hours, you will hear pilots making cabin annoucements and reply to ATC instructions on 121.50 instead of the ATC frequency. Simple case of not paying attention to the details.
And based on the fact that the aircraft has visited both of the locations with 361.3 within that past week...perhaps that is the source of the frequency choice. They never were close to Giantkiller airspace to even need to contact them or use that frequency for comms with GK
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