I dont really take the classification of the degree as too much of an absolute indicator of ability, and of course not all 3rds were created equal.

There was a guy two years above me studying engineering who got an ordinary (a pass, but less than a third) and was invited not to come back by my college. He then was taken by Imperial to do Mech Eng and and came top of the year. Obviously an ordinary here does not map to a 1st at Imperial, so what was the difference? Well the volume and difficulty of work was reduced a bit, but sufficiently that it made the difference between the fairly binary options of either sinking or swimming, and whereas he sunk here, he very much swam there.

So based on this one data point, it would seem that your attitude to dealing with the fact that you cant possibly be on top of all the material given to you is perhaps orthogonal to your innate academic ability, and where you are in this coping space (in the mathematical sense) makes a bigger difference than than if you just project down to the ability axis.

Also, its not all geeks who get firsts. In one of the projects Im working on at the moment my partner came top of his year, and hes frustratingly, sickeningly normal. His first does not mean hes a geek, but it does mean he can do whatever he wants, pretty much wherever he wants to do it. Firsts open doors.