Because I haven't the first clue! Dd is 5 days old and does a fine line in posset, sometimes an hour after a feed. Ds, now 2yo, was never sick or wasted any of his milk :)
Does posset signify milk they don't need? She can be rooting like mad but also bringing up the last feed. Does that mean she doesn't need more? She feeds pretty frequently and cluster feeds all night. She'd only lost 2 % on day 4 so is definitely getting enough! She also usually falls asleep during a feed but rarely comes off without encouragement. Could that be comfort sucking and not milk she needs?
It really is true that all babies are different!