Sue, with a simple metronome you would just set it at 50 bpm and know that it clicks, beeps, flashes, pokes you, etc., on the first and third quarter note of each bar.
Yes! That's what Strum Machine's timing amounts to. Whereas my naive elementary-school understanding and use of metronomes is to have it click on every quarter note in each bar. So if Strum Machine and my naive metronome are both set at 60 (=1 click per second), Strum Machine plays 1 bar in 2 clicks whereas I think of 1 bar as being 4 clicks.
So when I set SM to 100 and rip into Old Dangerfield, I'm playing it at 200 quarter notes per minute.