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often they are agents that manage social media for different businesses. they should be counted as separate

They are still one set of eyeballs and one brain to advertise to. Since the metric is mainly for the purpose of "how many eyeballs might see these ads" it still makes more sense to count them as one.

Feels like a quirk comes from the fact that a lot of prominent pseudonymous Twitterers, that they make accounts per communities and go extra miles to dis-link each ones, especially from real life identities.

Tangential, but twitter's customers aren't their users, it's their advertisers. The users are the product.