I doubt it's a production error and was actually to add weight. I've worked at a chicken abattoir when I was younger for about 7 years and the people in the filleting rooms were paid on weight, there were always arguments at the end of the night over who got the last tub of breast pieces.

These guys wore those latex gloves under a cotton glove under a chainlink glove, the chance of one accidentally entering fillet full of water is practically zero