Once in a while when I'm thawing and prepping frozen shrimp, I'll come across one that's quite a bit softer than the others. The flesh is a bit floppy and not firm like the others, and even the shell seems thinner and flexible. Other things like size, color, and smell are all fine. Is there anything wrong with these shrimp? Maybe they were sick or malnourished in life? They seem to be fine after cooking, indistinguishable from the rest.
If the shell is thinner that could indicate that the shrimp is younger, which would also explain the softer meat.
I think a good possibility is that the shrimp has recently molted, within an hour or two of being caught. The shells of arthropods are soft at this point to give the animal the chance to grow before it hardens again.