Parent comment said this correctly, but to clarify: the change doesn't actually increase overall inventory, it only increases "targeted inventory," i.e. you can now follow a jerky subscriber around to other subs, but the total possible impressions across all users is still the same it was.

Seems like Reddit could offer an additional option that limits display to a particular sub only. That way their bigger advertisers could still "spend more" to follow a jerky subscriber around but bold_panda could buy only /r/jerky inventory