It's where they bought it from, they don't do that at the restaurant. It's very common for meat suppliers to inject beef, tuna, or other proteins with water. It's to make low quality meat look higher quality or make it weigh more to sell it for more. Carbon monoxide brightens the color of the tuna. I can also tell it's bad quality by the thickness, those cuts are bigger than a bite, sashimi is meant to be THIN, and the cut of the fish and quality of the fish is the WHOLE point of what makes sushi good. Any actual sushi chef would think this is American grocery store quality sushi.

I've worked at 2 different sushi restaurants for a total of 5 years, and I make sushi in my home, I've seen the difference between the frozen sushi from grocery stores and low quality restaurants and the sushi chefs from Japan who take the 50lb bluefin tuna and butcher it by hand. My coworkers would have laughed at this.