Umm.. well...
I they both start "pen---" and then have "i" in them... and.... "c" can sound like "s" if it has i infront of it, so I guess it's all pronouciation.
Sometimes if I say a word, I automatically say another word related too it, but not the situation. Like...

"In disgaea 2 Etna Flonne... err, Etna shows up and---"

I'm not sure if that was the exact sentence, but it was me talking about Etna, and for no reason throwing in Flonne....