Found affecting 7.2 at latest fix pack. In calendar portlet add a new event and edit its description. This will bring up an instance of CK Editor rather than Alloy Editor. Insert a table with the default settings in the modal.
Note table borders are not styled.
Table Borders to be styled
Disparity between Alloy and CK in 7.2 (possibly 7.3 too) regards styling of table borders