The iMac Retina 27" has a NATIVE display resolution of 5120X2880. When the screen is set at DEFAULT it is 2560X1440 which is perfectly 4 times less pixel resolution. With Retinal aware applications like Lightroom and Photoshop the text areas use pixel doubling (4pixels act as one but with some kind of smoothing) to act like the 2560X1620 resolution display so the text is larger but the graphics areas (the photo) acts like it is a 5120X2280 so the zoom is 1 to 1 at the higher resolution. If you change the resolution to 2880X1620 than it can not (will not) do this fancy pixel doubling so both the text and photo display at the lower resolution, hence the larger photo when zooming. The best clarity of the screen is at the native high resolution or the 2560X1440 as the display does not have to interpret pixels. This is not a full explanation of what is happening in a retinal aware app but is the reason for the zoom discrepancy.
If you want the photo display pixel doubled when zooming than just chose 2:1 when zooming when using DEFAULT resolution. I don't know if you can turn this behavior of the two resolutions in retina aware apps off but I like it the way it works so never investigated that.
That's the clearest explanation I've seen and it seems to make sense. Is this explanation based on Adobe documentation or your experience/understanding?
This is based on an Apple related document I researched at one time when I had the same experience I tried to find it again so I could just link to it instead of bumble through an explanation from memory but couldn't. If you search long enough you will find more info in it. I really like the way it works in Lr and Ps at the DEFAULT resolution settings.