We've all seen a comment which says something like this
"Grand theft auto with horses" (Red Dead Redemption)
This, in my view is quite limited to what the game is really like. It annoys me. So, the game may be like another one, but it's a still whole new game. Let me pluck out a few examples.
Red Dead Redemption and GTA 4 - These are quite similer. Both are made by the same company (rockstar) both have the same genre. But GTA 4s Liberty City is bigger than the wild west. However, in Red Dead Redemption, you have horses which, comparing it with a normal car, but with Niko Belic at the wheel, is slower. There are many differnces which make it a whole new game.
Fallout 3 and Oblivion - Similer to the other example: Made by same people, same genre. Though Oblivion is medevil and Fallout 3 is bigger. Fallout 3 has a real date where Oblivion doesn't. Many differnces again.
Splinter Cell and Batman Arkam Asylum. - Strange example this, though it's been said that B:AA is splinter cell without guns. Wierd. Anyway, I'll leave you to figure out the differnces as it doesn't take a brain scientist to figure it out.
So that's it. Maybe these guys are trying to be funny or getting attention. But all games are differnt, even if they are similer.