Stems can creep up the steerer. Tightening the top cap after the stem can possibly help holding the stem down while not putting more preload in the bearings.
Maybe I should've been doing up the top cap more often on the Kinesis 4SD. It's headset would often loosen off until I installed a supersized knurled bung instead of their shitty one.
Why would you tighten the top cap after you've set the preload and the stem bolts? I thought you did top cap, then stem, then you're done?