Mike,

In this example, think of the "do" clef representing C on the piano, and we're only playing white keys.

EXCEPT that there is the "te" accidental, so now it's Bb instead of B.

That is making you hear it as if it is F major.


Why is it like this? Because it is.
You'll get used to it. In this mode, if speaking in contemporary music theory terms, I know that the "do" actually represents the 5th of a major scale (as we know it today), and the accidental (which is common for this mode) represents the 4th degree, wanting to pull down to the 3rd.

Some chant scholars will try to discourage this kind of "translating" from one notation to another. I don't think much of it... it winds up sounding the same either way, and eventually you won't even be thinking of them in these terms anymore. Over time this stuff makes more sense - the more that you sing it, the more comfortable it becomes. ;-)