Formatting Message arguments

Can anyone explain why the first of these expressions works as expected while the second does not? What is the difference between "nu开发者_如何转开发mber one" and DisplayForm@"number one"?

Block[{$MessagePrePrint}, Message[f::event, DisplayForm@"number one"]]

Block[{$MessagePrePrint}, Message[f::event, "number one"]]

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The error message is a clue:

An unknown box name (Times) was sent as the BoxForm for the expression. Check the format rules for the expression.

I presume $MessagePrePrint is low level, like $PreRead, and its output needs to be a valid box structure.

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Works on my machine (v8.0.1, W7-64):