Under the topic "Vertical Terminations"
Standard Vertical Terminations
Exhaust Vent Terminations
The 2" vent must be reduced to 1-1/2" for the last 12" for the 60,000 and 75,000 models.
That may be why you're not having issues with the excessive 2" length because the termination is too large. So, do two wrongs make a right?