No it is not just you. Several other people make the same mistake because they do not understand logic or the rules/strategies of debate. Let me school you on how your logic is faulty:
If the subject was Bill Belichick as a head coach, then the sample size would be 26 years. Taking the past 15 years out of that 26 year sample would be fairly reasonable. It would not be the final word, but at least it would be a reasonable argument.
Unfortunately (for you), the subject is Belichick as a head coach without Brady. The sample size is 7.3 years. Taking the past 1.3 years from a 7.3 year sample is very weak reasoning. You would lose a point in a formal debate every single time when your opponent gave the full 7.3 year sample.