I've seen this issue just recently brought up on all the major cover sites, under various user names. boemba do you also post under the names Aristoles and soekie?
There is no "real" cover size. Retail covers are typically designed by an in-house studio or shopped out to a private studio but in either case the template size is dictated by the replication plant. In some cases, the artwork is entirely made by the replication plant's own people. Do some searches for replication plants and they all offer their templates for download, and the sizes all differ by a mm here or there.
Amaray can suggest whatever size they want but the fact of the matter is that the difference between 3240x2175 and 3224x2161 is negligable. You can easily cut off this much of a margin by accident when trimming out your cover. Retail covers differ in size by more than this.
If there is no industry standard then all that matters is that the cover scene set an appropriate scene standard, and that is 3240x2175. I believe this size came about simply because it was the size set by dvdcoverart which is what kinda started the whole custom cover craze for the masses. From then on it just stuck. It clearly works as a standard. It fits the case and it seems that 99% of established cover artists use it. So there's no reason to change it. As for whether alternative size are accepted on this site I guess that depends on how leniant the cover approver(s) want to be, but its definitely not an allowable cover size according to site rules.