3.13 CONNECT Payload - "MUST" versus "is" for flags

For ClientID, "MUST" is used:

Line 1289: "The ClientID ... MUST be the first field ..."

but then for all other fields, the language changes to:

Line 1315: "... the Will Topic is the next field ..."
Line 1319: "... the Will Message is the next field ..."
etc.

Why not, "... the Will Topic MUST be the next field ..."?