Real world explanation is probably that JEJ was drawing more inspiration from David Prowse’s delivery based on what we heard of him on the Tantive IV. He then made the role more his own as the films progressed.
It’s also likely because during the making of ESB they wanted to emphasize the cyborg angle more. Originally during ANH’s production, only his face was burned (his mask was custom made) and he was wearing an armored space suit with Sith robes. In post-production when Ben Burtt came up with the breathing sound effect, George Lucas thought it made Vader seem cybernetic so he decided to go with that, hence the more robotic look in ESB and ROTJ.
Once they honed in on the “more machine than man” aspect, part of that was the electronic filter added to his voice (first introduced in ESB), as well as his colder line delivery.