the performance of the Mac Pro as a cheese grater is unsurprisingly disappointing.
Expectations aren't necessarily predictions.
MW has three relevant definitions:
1a : to consider probable or certain
expect to be forgiven
expect that things will improve
1b : to consider reasonable, due, or necessary
expected hard work from the students
1c : to consider bound in duty or obligated
they expect you to pay your bills
The new Mac Pro is likely disappointing expectations that fall under definition 1b or 1c. It is considered reasonable/dutiful that a cheese grater should be able to grate cheese. The unsurprising part is that computers are in fact not cheese graters (Even if they look like them) and as such are not predicted to be able to grate cheese well. They failing general expectations, rather than predictions tailored to their specific circumstances.
Another situation that comes to mindcould take place at a Grocery store. Manager A has a particularly difficult employee - Cashier B - who continually under-performs the company standards. Cashier B is disappointing with respect to the company standards, and consistently disappointing. Therefore Manager A would be accurate in saying that "Cashier B was unsurprisingly disappointing".
Overall, something can be "unsurprisingly disappointing", because the expectations that are disappointed aren't necessarily the logical predictions of the person that felt "unsurprised".