In mathematically oriented code, long variable names make code less readable, not more. Mathematical expressions become long and difficult to parse, and the variables often have no intrinsic meaning, so trying to make up 'meaningful names' is pointless.
Some things are just very common in math notation, enough that people recognize it better than fully named variables. Things like uppercase letter for a matrix, _T for a transpose, etc.
For someone with maths training (be it years in undergraduate engineering or whatever), one does get quite used to one letter names. Note that they always should have limited scope.