sondenso Mentions it well but I'd like to simplify the thought process
Consider parathyroid glands as a Positive sign (+) and consider adrenal glands as a Negative sign (-)
Now we all want to have +ve factors in our lives and discard the -ve ones. This is precisely what is happening. When we remove parathyroid glands ( a +ve factor) we see something bad happening (calcium level falls sharply) So now we have only a -ve factor ---> very bad situation
BUT when we also remove the adrenal glands, a -ve factor, something good (or rather something less bad happens) i,e.. the calcium level falls bet less sharply
To sum it up :-
No parathyroid glands --> only negative factor present (adrenal glands) --> sharp fall in calcium level
No adrenal glands --> only positive factor present (parathyroid glands)--> no fall in calcium level
Combining both --> No (+) and no (-) --> middle ground i,e.. fall in calcium level (but not sharp fall)
TAKEAWAY - Create analogies in your head and relate to the argument
1) no positive factor and only negative factor present --> VERY BAD
2) no negative factor and only positive factor present --> VERY GOOD
3) no positive and no negative factor present --> middle ground ( better that (1) but worse than (2) )