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Why do these Select modifiers behave differently? Is it an Access bug/oddity?

No, that's how SQL is defined. The empty string and NULL aren't the same thing. The empty string means there is a definite value there, which is blank, whereas NULL means there is no value there. Similarly, zero and NULL aren't the same thing in numeric fields.

If any of your 24 blank records appears among your 79 NULL records, then that's an Access bug. If your data is so dirty that both "" and NULL have been used with the same semantic intent, you can either clean it up or use (Field Is NULL or Field = "").

Peter