Good question, but the explanation is wrong. SQL Server always parses each query.
From the justification of what the correct answer is:
Therefore a single value in the WHERE... IN clause can be surrounded by multiple pairs of parenthesis. Multiple values cannot.
Why then do these work ?
WHERE Musician_ID in (((1)),4)
WHERE Musician_ID in (((1)),((4)))
Because the syntax for IN is: IN (expression, expression ...)
One set of parentheses around a comma-seperated list of expressions.
Of course, ((((((((4)))))))) is a valid (but rather pointless) expression.
EDIT: Or I could just have scrolled to the end of the discussion first, and seen that Carlo has already posted something similar.