When you query Reach at the campaign, ad set, or ad level, Meta does not deduplicate users across campaigns or ads.
If the same person sees multiple campaigns, they will be counted once per campaign, ad group, or ad.
If you request the Reach with a query at the campaign level, you get the sum of reach at the campaign level, not the reach for the account. The same applies to ad sets, ads, or any breakdown.
The only way to get the correct reach at the account level is to request it without a breakdown.