Good catch @Laura Paris!
I can't say for sure without more details but, my guess is it has just created an Atlassian account for you behind the scenes without you noticing, has copied your HC user details (name, title, password, etc.) over to Aa, and now is linking them for you. The purpose of doing it this way as far as I know, was to make Atlassian account a transparent and seamless part of your Atlassian experience.
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