Actors’ unions have an “official” version of each performer’s name. This is generally expected to be unique, for the purpose of keeping professional credits straight, though there are exceptions.
Consider the example of Harry Dean Stanton. He was credited as Dean Stanton for the first 20 years of his career. Why? Because this guy was already an established working actor, and Harry Dean decided to go by his middle name to avoid confusion. Later, he switched to H.D. Stanton, and finally to the full first-middle-last version we know today. For some actors, using an alternative name like this results in a personal annoyance, when your professional moniker doesn’t reflect how you like to be referred to in the day-to-day. His fellow cast members see him as “Dean” on the call sheet, but then he shows up on set and has to tell everyone, “Call me Harry, I just use my middle name Dean for my career.” When he ultimately switched to a version of his actual name, this probably came as some relief.
Another possible example: Ever wonder what the “F” stands for in “F. Murray Abraham”? Nothing. He was born Murray Abraham but added the F when he became a professional actor. He says it was in honor of his father, but I would speculate there was already a Murray Abraham on the union rolls.
(Sometimes it’s not about a unique name, sometimes it’s just for marketing or ease-of-pronunciation purposes. Lea Michele Sarfati chose to truncate her family name for her professional career, so as not to limit her casting opportunities. And of course Tom Mapother probably wouldn’t have become a giant star if he hadn’t changed to Cruise.)
Anyway, point is, IMDb listings use that singular official name as the heading for the actor’s page. If the actor is credited by another name, even one with a small variation, that gets noted, as you observe.
Go look at Harry Dean Stanton’s IMDb page, and scroll down to the 1960s to see this in operation. And here’s another example: At the beginning of his career, Laurence Fishburne went by Larry. That’s how his name appears in the credits for his early appearances, such as Apocalypse Now. And the IMDb entry for that movie reflects this. But in his personal life, he prefers Laurence, so eventually he changed his screen credit, and now that’s how everyone knows him.
So, yes. “Richard S. Castellano” was the preferred syntax for that actor’s screen credits, but for whatever reason a different version was used for The Godfather, so IMDb shows the variation.