Two emails received from the same sender and only one considered as suspicious by Gmail

Within 2 minutes difference, I have received in my Gmail two emails sent from exactly the same sender. First message was marked as suspicious:

While second one was not:

Is this some kind of a bug (either in Gmail or i.e. in sending server that did not include something expected by Gmail in first message)? Or is this pretty normal and obvious (meaning that those two minutes that passed or receiving second message caused Gmail to learn that this particular sender is actually "safe")?