Why do you say that _italics_ and *italics* are semantically
different?
What do you mean?
The parenthetical hinted at it:
(I tend to use the former for titles and such)
There's a convention at work here, namely that italicized book titles and similar are often underlined in plain text — and in plain text without backspaces this is represented as leading and trailing _. So there's a semantic (i.e. content) difference even though the generated markup is the same.