Comments on the original post indicate that if you do 6/7(1+2) then you get 2 as well and the iPhone is just throwing away whatever digit is in the position of the 7.
It seems that pushing "(" doesn't push the entered value onto the stack, even though it's still visible on the screen. "6 / 2 (2 + 1)" becomes "6 / (2 + 1)".
The calculator on WP7 does the same thing, but is a little more explicit - it shows the current expression as it is being built, so you see that while the first 2 was entered, the final expression is 6 ÷ (2 + 1)