I've found that a lot of code is written by mathematicians or physicists, and on paper the formulas all have one-letter-variables, so they will copy that one-to-one when they implement it in code.
For instance, an electronic engineer will know that the formula for calculating a charging voltage of a capacitor in an RC circuit is:
U = U0 * (1-e^(-t/(R*C)))
So in code, I will reflect exactly that:
double U = U0 * 1-pow(e, 0.0-t/(R*C));
Every non-electronic engineer is going to look at that and go "WTF IS THIS CRAP".
I've had moments where I've done the same thing at first glance, but as soon as I research the algorithms being used, the variable names begin to make a lot of sense.
I mean, when you are writing hundreds and hundreds of lines of code, it gets annoying typing in long names.
One word: auto-complete.