What university are you at? Every university classification system is different, but many have a clause whereby if a student has performed well over the course of their degree and is only brought down by one assignment, then they are awarded the upper classification.
Many other universities have a system whereby they award you the upper classification if you are within 1% of achieving it.
However, I fail to see how someone who has achieved 80%+ in some things can have had their average reduced so much by one 2:1 grade. A 2:1 is still a good mark on a piece of work, it seems bizarre that someone who has achieved firsts on everything else can get a 2:1 on their dissertation and then be awarded a 2:1 overall.
Was your dissertation a low 2:1?
Or is it weighted significantly higher than the rest of the degree elements?
Or was your average actually just over the first class boundary (70-75%)?