There is no such thing as a "macro" if its below 1:1 Shame on Zeiss.
1:1 ratio is a subjective value. You get more magnification with 1:1 macro on crop, than with 1:1 macro on FF.
Something like A7R (36mp sensor) allows you to crop a lot and get the same magnification of a 16mp crop sensor. So, technically, 1:2 macro on 20mp crop provides similar magnification to 1:1 macro on 12mp FF.
1:1 has nothing to do with megapixels; it is entirely governed by optics and means the physical size of the projected image equals that of the subject, ie a 1cm tall object results in a 1cm tall projected image.
Due to the cropping factor of an aps-c sensor though, you are effectively blowing up the projected image. So this means that for a crop factor of 1.5, a 50 mm lens will become 75 mm equivalent, and at 1:2 magnification becomes 3/4 magnification.
Well, lens magnification has nothing to do with megapixels, but the amount of information you get from it depends on the number of pixels you put on each millimeter of the projection. I may be using wrong terminology here, I'm sorry for my bad english. IMHO, on different sensor sizes (FF, APSC, m4/3) 1:1 ratio has different values, as well as with different resolutions of the same sensor size, because, when it comes to the final (real macro) image, everything matters. At those magnifications I mostly care about the information I can capture with the final image in mind (on big display or print), not how big it looks on my camera LCD or in a smartphone snapshot gallery.