The residual water level in my downstairs toilet bowl often goes down very low. Why?

If it initially fills to within 1" of the stand pipe top and then drops low later your flapper is likely leaking/seeping. If it never fills completely your ballcock float rod needs adjustment. Which case do you have?

Bal, It's thr bowl level that goes down, not the tank level. So, if it doesn't do it all the time we can rule out a crack in the bowl. A blocked vent would do it. Do any of your fixtures gurgle when you flush or drain? Under what conditions does the level drop? Back to you, Tom