audio mismatch

Hello.

I work with singers. I "thought" one of my students was hearing what I played on my keyboard one full step lower than I played it. But when I listen to the playback of the recording, she is singing exactly what I played. In the live session I am hearing her sing it a step lower. I can tell something is wrong with the audio, because my voice sounds unusual, though the keyboard sounds right. Hard to explain. I was blaming all of this on her end. 

The lesson I gave immediately following that lesson matched up perfectly.

The prior week, the student with whom I had the mismatched audio and I had perfectly matched audio. 

Please tell me what to do to avoid this problem. Thank you!!!!