This is just a guess but the 27 inch may be treated differently with respect to system upgrades. I thought that the new system may be too complex for a slow spinning HD that came with the smaller iMac. The new systems seem to depend on speedy SSDs. My thought was that with my SSD upgrade it would work. I am assuming that your 27 inch iMac has a HD with a much faster rotational speed (as my 27 incher did before it crashed). Honestly, I don't know.
However... I have an iMac that is awesome. Startup went from 2 minutes to 8 seconds. It has a good I5 quad core processor a 1 TB SSD and 16 GB RAM. As long as I can get security updates, I am perfectly happy with it.
There are lots of knowledgeable people on this forum that can explain with more clarity.
Regards,
S