Chemical boils at almost 10 degrees below it's BP??

I might be dumb, but I need an explanation...

Hi all!

I'm a silly little college kid, and I'm in a General Chemistry lab.

Our latest lab was over identifying boiling points. We were given an unlabeled chemical, and had to find out what it was through its solubility and boiling point. But my boiling test was a bit.. odd..

My first trial was 99°C, but my second was a whopping 90°C, which was weird. At first.. I chalked it up to maybe the test tube containing the unknown being wet, messing up the first trial. But no, mystery chemical ended up being 1-Propanol, which has a BP very close to water.. meaning somehow the 1-propanol boiled early? How could this be?