When Ichika said that he kissed before, she may have meant when he was sleeping during the class trip and she kissed him.
I suspect the way it was going to go was
F:"What are you saying, I haven't kissed anyone yet"
I:"Surprise, I kissed you while you were sleeping"
The fact that Fuuts blurted out that he did as he was remembering the bell kiss caught Ichika by surprise since she didn't witnessed it and couldn't have known about it, I mean look at how she was drawn when she asks about who it was, she has an expression that screams "I'm gonna murder whoever that was" on her face.
This is all Negi's fault, that kiss back then was probably planted for this specific reason, so that if another kiss occurs with no known culprit as is the bell one, Ichika would be the only one who can inquire about it without including or excluding her. Even if it wasn't her that kissed him under the bell, she still has a kiss on her tally, even if Fuuts is not aware.