In fact, speaking of overexplainy sequels, it technically isn't even the first time. From Vader's pespective this is VERY reliably the way to make Kenobi pop up from whereve he's hiding. Grab Leia, get Kenobi. Sure Luke doesn't know who she is, but Vader doesn't know that.

The droids are weirder because Vader would have thought Obi Wan had them all along, but they were with Leia and they are the ones who went to warn Kenobi. Kenobi was coming to Alderaan to deliver the plans, not looking for Leia. This looks way more normal to Vader than it actually is for us.