Why do HTML entity names with dec < 255 not require semicolon?

Why do HTML entity names with dec < 255 not require semicolon?

In a plain HTML document &pound (dec 163) renders as £ without needing the
;, whereas &oelig (dec 339) will only render a œ with the semicolon. It
seems that every html entity with a decimal value under 255 will render
without needing the semicolon, both in FireFox and Chrome.
What gives?