I think they just sent out the email in batches. May be in the ordered when the random selection was generated. So again, random batches.So they put people in batches?
Maybe first week go into pool 1. How many divide by total number of applications x 100. Then they is the percentage of spots for that draw. Then next day for next week.
As some have suggested, IRCC is probably trying to avoid overloading traffic and the email being considered as spam by some mail services.