FTP Increase - What happened!?!

Team- I am BRAND new to all of this so it is most likely user error. Recently did a Ramp Test and scored 162. I then did this “Ironman” course and it was the single hardest workout I have EVER done. What the heck happened? My 4.5hr watt average was 164. I am just baffled. I could not get the watts any lower…What am I doing wrong?