Interesting oddity editing runes in champ select

Recently, I decided to switch to playing teemo top, after enemy picked ornn.

realized I didnt like my existing teemo page. Started to edit another one. Hit save on it. The editing thing didnt show very clearly that I had one of the thingies missing. So it didnt take for my active runepage. Here's where it got interesting.

It did NOT select my previously selected, non-edited rune page It did NOT randomly select a missing rune for me It did NOT randomly select one of the special 4 pre-made rune pages It did NOT randomly select one of my other many rune pages.

It somehow pullled out of NOWHERE, a mystery rune page. I went back later, and tried to find any match in my rune pages. Nothing.

The funny thing is, it turned out to be a GOOD rune page for the match up! (I hesitate to mention the specifics, because I dont want to invoke some flame war over different peoples opinions on whether it was really"good" or not. It worked for me!)

So, Im wondering if other people have noticed this, or this is a "bug".. or this is actually a stealth intelligent rune picker. If intelligent, would be good to know the mechanism behind it.