Star memory costs wrong?

I've had a bit of extra stamina recently, so I've been resuming my star sea recolor grinding and I noticed that despite being listed at 16, it's only been taking 12 star coins instead of 16. I noticed it yesterday because my mental calculation didn't come out right and I just kinda assumed I did it wrong, well today I definitely was paying attention and even spent a few diamonds for extra coins just to make sure I wasn't wrong and yep from 18 coins I was left with 6. I didn't see any mention on the wiki, and I didn't remember it when I was working on the pink recolor last year so was it some kind of oversight or what.