My feeling coming out of some conversations a few weeks ago is that some expectation exists for us to innovate or bring something new to the mechanics with each installment. I haven't read through all the reviews yet, so I don't know how pervasive that anecdotal feedback was.
I think it's perfectly fine to not innovate on every installation, especially since this is a DLC of sorts.
I also think that you have to take your lack of innovation when considering what scores you'll get, because a review score ultimately reflects the general experience with the game, and of course seeing "old tricks", even if incredibly well polished, it's just not gonna dazzle as much as new tricks will.
The fact that Resident Evil 5 polished up the gameplay of Resident Evil 4, doesn't mean it'll impress as much as that one did, when it came out, and that's gonna be reflected in review scores.
I don't think it's weird at all, seems pretty natural, in fact.
Doesn't mean devs should obsess with innovating at every turn either, not every game should aim to be a 10 (whatever that means) or reinvent the wheel.
Not to mention, trying to be innovative also comes with a lot of risk, as that's where you find a lot of shitty games, too.