Airbubba, misset alts is the most plausible explanation to cover the actual and ADSB profiles. My comment about lower than standard pressure would have made the error even easier to make, simply forgetting to select QNH at transition, but the error can still occur from a misheard or mal-transmitted QNH. Whatever the cause for altimeter error, it's still the leading cause for non-precision app accidents and fits well here as something to be investigated. It may, of course, have nothing to do with this event...