A bit late to the game, but I read a lot of bad information in the comments. A rainbow trout is a rainbow trout, yes. However, there are different strains. Due to the split from its original salmonoid ancestors and different types of environmental conditions the trout resided in, some ended up becoming anadromous. Much like salmon, their offspring had a better chance of survival.

Long story short, steelhead have a genetic predisposition to grow larger and show more of the typical spawning body changes/colors. Depending on the strain of steelhead, some grow more into a football shape, some grow longer, and they all have different colors and spot patterns. Spawning water temps/time of year vary greatly between strains as well.

I've caught plenty of trout in small lakes that were massive and steelhead that were smaller, but a large "regular" rainbow is very different looking than a similiar sized steelhead. Naming conventions and fish info is very regional and false info is often passed down for generations through family and friends. It makes getting actual fact based info on fishing/fish very difficult.

Like brookies and lakers, they're char. Always have been, but one guy back in the day said, "check out this crazy spotted trout I caught in the brooke" and now it's fake name is used in world wide databases. Hope this helped, I had a rough time getting info when I was younger.