Labels at the big box stores are not always some sort of conspiracy or a blatant lie to make a buck. In fact, I believe that almost all of them are a consequence of ignorance and such. So many people that work there and also the public at large still think/believe that fishes grow according to the size of their tank... or that fishes in captivity are always far smaller than their wild kin. That's how such labels come about, IMO.
And sometimes it can be true - see this thread:
http://www.monsterfishkeepers.com/forums/showthread.php?505668-ID-sharks-theories&highlight=theory - some of our own MFK-ers report slow- and small-growing IDS not exceeding 1'.
The same is occasionally observed for most other fishes, whether it is a bad keeping, dwarfism, bad genes, different species or sub-species, altogether improper ID (e.g., piraiba vs. capapretum), etc. etc. etc. This plus the fact that people tend to collect knowledge that conforms to their ideas and goals also can contribute to the prevalence of such labels.
The pacu in the photo is still small. They grow to 36" and almost 100 lbs. This guy is probably a bit over 1' and ~5 lbs.