They both do the same thing.
He split up the 3rd and 4th lines, I don't know why, but if you read it it's the same. The difference between member and party_member is just the name, it doesn't change a thing. Yeah I wrote it, but so did he.
It's well known among scripters that to make a variable equal something you use $game_variables[VARIABLE_ID] = whatever, and to reference the actor's ID you just need to use that global, $game_party.members[ACTOR_ID].id. Doesn't do anything different.