all monarchs are equal...
but precedence is based on how long they have ruled...
see for example, Queen Elizabeth's Diamond Jubilee Monarchs' lunch
you will notice that the most senior monarchs (by length of time ruling) sit in the portrait (the more senior, the closer to Queen Elizabeth in the centre)...the Emperor of Japan (sitting) is senior in precedence but other monarch's have reigned longer (and thus sit closer to the host Queen)
all of the world's reigning royal families sent representatives to to the Monarchs' lunch except Spain, Oman, Cambodia and Bhutan
also, some ex-monarchs attended (Kings of Greece, Bulgaria, Romania)
Jubilee monarchs' lunch: the full guest list - Telegraph