sql 'not in' vs 'not exists'

I am trying to convert a sql that has a sub query with 'not in' clause to a sql with 'not exist' for performance reasons, but i get different results in both my queries. Please help me understand and get the right query that would give me the same results as the 'not in'  query.

NOT IN sql :

		select 
			id, 
			'Patient',
			'Missing Name',
			null,
			'l_c_claim',
			'p_l_name',
			pat_last_name
		from wt_l_c_bill		
		where p_l_name IS NULL
			and id not in (select b_id from app_b_r_v_item where validation_category = 'Patient')

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'NOT EXISTS'  sql

		select 
			id, 
			'Patient',
			'Missing Name',
			null,
			'l_c_claim',
			'p_l_name',
			pat_last_name
		from wt_l_c_bill b		
		where NOT EXISTS (Select 1 from app_b_r_v_item v where validation_category = 'Patient' and b.id = v.b_id)

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Thank You