Why can’t TestCafe expect a expression including both a Selector and a const value?

I have a JS code of TestCafe which could not get the expected result (NaN)

await t.expect(Selector('some-expression').count - someConstVar).eql(1)

However if changed to following, it would work well.

await t.expect(Selector('some-expression').count).eql(1 + someConstVar)

I felt the first one questionable and changed it to the second, although it worked, but just curious, why?