Sending empty prompts still costs about 8-11 tokens?

When I send an empty prompt with [“role”:“user”, “content”:“”] the response[“usage”] payload shows that AFTER subtracting the completion tokens, the cost of an empty message still costs about 8-11 tokens (amount changes randomly when I resend the empty message). I have calculated this by subtracting the cost of resending the old data.

Can someone please explain to me why this is the case?