Why does Smollett pretend to know what Silver is talking about when he says in Chapter 20, "He wasn't dead when I got round to him, not he"? What do you think really happened?

Captain Smollett is out for his own interests.... he and Silver have the same goal, and that is to find the treasure. Smollett isn't going to give away what he knows or doesn't know, he's merely trying to gage what Silver knows. It's a game of wits.

From the text, we can infer that Ben Gunn visited the pirates' camp while they were drunk and managed to kill one of them while there.