Slightly wrong product numbers.
The 26842 is, as you say, of an un rebuilt model but it has an early MFX sound decoder which was universally slated for being very poor sound wise.The chuffs were far to fast and the sound was tinny. The recently released 26843 presumably has an updated sound card fitted. I would think the differences you can hear are most likely to be because of this modification rather than any difference in the digital recordings between locos..