They send the file to regional Canadian high commissions based on the applicants’ current country of residence. If you provided an address in Canada as your parents current address, then their applications would be sent to Ottawa.
This is pretty apparent since people whose applications got sent to Ottawa have their parents on Super Visa in Canada. I’m assuming that you guys sent the PGP application after your parents are already in Canada on Super Visa, which forced you guys to provide Canadian address as their (your parents) current address.
I provided outside Canada address for parent, but it still was forwarded to Ottawa