Possible bug in round bed limits

My printer has a round bed a little over 300mm in diameter.

My astrobox printer configurations specifies the max print diameter is 280,
which I intentional set a little small.

However an attempt to print a 220mm ring shape the slicer (cloud slic3r) I am told
that the object over sized…and recommends reduction to 195mm.

I do give a 10mm spaced skirt but I would think then the object is really 240mm still
within the 280mm limit.

Maybe I am missing something or maybe slic3r is just a little conservative…and I can override the suggest reduction so not a problem really but thought I would mention
it if the part of the code is being revisited in an upcoming release.

Thanks