Wouldn't the secretions from the sweat glands on his hands massively affect the results? At several points on the video, he touches the SD cards without even having gloves on; from the time behind the scenes, where he must individually touch every SD card, a lot of substances probably build up on the SD card.
The sweat glands on the hands (which are found on most parts of the skin) are the eccrine sweat glands, which secrete (from Wikipedia; it's the only thing I have right now):
Many of those substances don't evaporate, and I suspect that the pores in the plastic would fill with those substances.
I guess it could vary depending on what order he did it; if he went from filling an SD card and recording its weight, to deleting everything from the SD card, the 2nd step would have more collective residue, and as the mass is lower, it would prove that the experiment is true (not too scientifically, however), but I can't tell from the video.
I'm very surprised at the large change, which what I explain above might be the reason.
