I don’t think it was just one thing but many small things.
Did the captain understand the difference between a tropical cyclone and an extratropical cyclone?
From the VDR transcript both the captain and mate seemed to expect max about 40 kts. Both expressed surprise when they got the bridge.
The mate and captain did not appear to take into account possible forecast errors.
Captain had high confidence in his abilities and dismissive of the concerns of the mates,
Another thing is the illusion created by using the BVS on the computer. When the weather is plotted by hand and an eye kept on the weather things seem much less certain then on the computer screen, just like the mates using AIS for collision avoidance without understanding the limitations and not seeking other sources of information.
A display like this will fool you, the “feel” you get by hand plotting is different.
Mario called it the illusion of control. I spend a lot of time clicking through the 6 hour forecasts, watching the lows move as the little ship icon advances along the track. Far different then reading the Sat-C text etc.