Because you're composing the reply in Gmail. They populate the To and CC fields of the reply using their own rules and settings to choose content from the original message's fields (From, To, CC, Reply-To, etc.).
In the case of Reply Only To Sender messages Groups.io does not insert a Reply-To field in the original message, so Gmail (and other receiving email interfaces) would normally reply to the From address. I don't think it would help in the Reply All case if Groups.io did supply a Reply-To field - I think Gmail (and others) would include that content in the reply but would likely still include the original message's To address (the group).
I haven't tested it, but if that is indeed the case I do wonder how that is achieved in the face of Reply All behavior.
My guess is that it doesn't, that the Reply All does in fact return to the group as well, and it is the lack of a bounce that made you think otherwise.