Emacs Calc (not unlike much of the rest of Emacs) is a little ... special. By default, it calculates 4/2*2 as 1, because multiplication has a higher precedence than division.
Wow. That's actually quite bad.
I think there's also some confusion about whether the / symbol is actually being used as a single line equivalent to a fraction, which might introduce some ambiguity? I forget. The number of ways that people can misunderstand things is infinite, so it can be hard to keep track.
Anyway, parentheses are cheap and misunderstandings, even when the other party ought to take the blame, can be expensive, so I've taken up the habit of being generous with the things.