Without doing it myself, and noting many other parameters, my gut would be that the cat was doing a better job on the second test. Possibly overall engine temp higher, and more consistent, ie; oil/coolant/cyl block/cyl head all equalized, and intake air also steadier. Those of course, all depend on how long the vehicle was driven before the first test, and how long after the first test the second was.

The gas cap, under most normal circumstances would not affect tailpipe emissions much, as you mentioned. Under some rather creative scenarios, the lack of a sealed tank could mean that evap purging was unrestricted, and hence able to flow more volume, thus affecting the fuel mixture. But that's pretty unlikely.

MV