Every university is different I'm afraid. Check your handbook, there may be an explanation of how they decide classifications. At my university, if your average is 68.5% or higher, and you have a majority of firsts in your modules (so for instance 4/7 modules must be firsts), then they round you up to the First. However, if you don't have the required number of modules at a First, they will leave you on the 2.1 - a friend of mine was left at a 2.1 with an average of 69.7%, sickening. The only way to know for sure is to check your university rules!!