That is interesting, perhaps a synchronisation error? As if the actual comment wasn't available yet but the email already got sent.
And I've seen it in 100% of your replies to me, when I've never seen it for other replies. So I wonder if it's related to how you use HN
Edit: I truly did not believe you, but it appears to maybe be the “delay” feature [0]. Users with a delay set might show the asterisks until the delay passes.
My apologies.