hmm, you could be overthinking it and it could just be as simple as experimental error or instrumental error, human error, etc.
Examples could be the system miscalculated, the oxygen mask wasn't sealed tight enough to the subject, older machine with defects or something silly like that.
Here, I found this in like 5 seconds by googling "find the error in submax testing", lol, it looks helpful
http://www.vacumed.com/images/ErrorsinVO2testing.pdfIf you click the click it gives a error.. highlight it and copy/paste in search bar and it works, or google exactly what I said and it's like the 3rd link down
Hm that was easy, I guess I was over thinking it.