I think it is different every year.
It does not appear that HBS admissions have fixed plan on exact numbers for the invites for first and second wave.
Since the invitation release date is fixed, I think they just sent out invitations based on the date and not based on the amount of application they selected for interview invitation.
If that is true, then it must have taken longer time for the adcom to go over applications this year than last year.
I surely hope you are right!