Calculating number of validators on dune

Question

to find total validators on eth , i am using below logics :

SELECT sum(amount_staked) FROM staking_ethereum.deposits as a,
SELECT sum(amount)/1e9 FROM ethereum.withdrawals as b

and my output is (a-b)/32 , am i making a mistake ?

I saw some query where hildobby did

CASE WHEN amount/1e9 BETWEEN 20 AND 32 THEN amount/1e9
     WHEN amount/1e9 > 32 THEN 32
END AS amount_full_withdrawn 

so instead of subtracting amount I calculated above, he subtracted amount_full_withdrawn But I am unable to understand this logic.