There has been upsets in the NRC final before now. How many people backed Karl Harrison's Batley side against Widnes? and Doncaster reached the final (albeit to get drubbed by Salford) when they were in National League 2 as it then was.
Knock out rugby can often throw up oddities, so while a Sheffield vs Leigh final is what people may think is inevitable, who knows, they might even meet in the semis....