(Note that the explanations below are scholastically inaccurate.)
To be exact, there are a few variations in the system of ON reading: Go-on(呉音), Kan-on(漢音), To-on(唐音) and kanoyo-on(慣用音).
http://en.wikipedia.org/wiki/Go-on
http://en.wikipedia.org/wiki/T%C5%8D-on
In simple terms, Go-on is old ON which was imported from China or other contries in 5th century. Kan-on is of 7th century and To-on is of 12th century. 慣用音(idiomatic ON) is "Japanized" ON, or arranged ON to fit for modern Japanese in around 19-20th century.
In 皿's case,
Kun-yomi: sara
Go-on: myou, myau
Kan-on: mei
Kanyo-on: bei
Not all kanji has all of kun, Go, Kan, To and kanyo since some of those ON don't exist, or vanished and are not used today. The judgement of vanished or not vanished depends on editors of each dictionary. Japanese wiktionary listed Kun, Go, Kan and Kanyo for 皿. English wiktinary listed only Kun, Go and Kanyo. Maybe your ditionary considers all of them except Kun were vanished.