Why Does this Happen in the Rune Page?

So I play Lux and in the current meta, I like to run biscuits for mana in lane. However, when I'm playing Lux and vs a Yasuo, Zed, Talon, I take stopwatch instead so I can save a little on building Zhonyas to account for less CS and because Assassins.

Why is it that in the Rune page, I can swap biscuits for stopwatch just fine while keeping cosmic insight as my second secondary rune, but when I try to swap stopwatch to biscuits, it replaces cosmic insight? This has caused me to run biscuits + stopwatch a few times which isn't good for item slot reasons. If I click on stopwatch and click a rune presumably in a different row from cosmic insight, it should change the stopwatch, not my other rune. It makes no sense because it works fine going from biscuits to stopwatch.

Can this be changed? It's annoying to remember to switch to biscuits but then time runs out and my runes are wrong.