If I sent an email on a certain day, then why am I receiving RUA reports for that particular email after 48 hours?
Aggregate reports for a certain day are sent on the next day. For example aggregate reports covering emails sent on the 25th, will be sent on 26th (at various times throughout that day depending on the reporter). Therefore those emails won't be visible until then. RUA reports are sent once a day by most of mail receivers.