When a constituent is on two event invitation lists, if they accept one invitation they show as registered on both invitation lists. If they decline one invitation, they only show as declined on one invitation list.
CRM behaves this way because to decline both invitations with only one constituent reply would undermine the ability to have the same constituent on multiple invitation lists. If a constituent accepts an invitation, they show as registered on both lists because it is clear they intend on attending. If the constituent declines one invitation, they should still have the ability to accept the second invitation because they've been included on two invitation lists.