Upair input issue

[Coccinelle]

I am more like a beginner at SSB and was currently working on aerial combos with Falco. When trying to rapidly repeat short hop Upairs I had a weird problem: sometimes my input would result in a Nair instead of Upair. Obviously my input is bad but I don't understand why because I am using the Cstick for tilts (and aerials). I do not understand what (wrong) input with the stick + jump can result in a Nair, since as far as I know, neutral moves can be done with A button only, not with the Cstick. I know it is a pretty basic question, any explanation would be welcome.

 

[Coccinelle]

Please delete
Moved my question to the Q&A thread