@murrayjames,

变量 and 变数 are interchangeable in Math or computer science. For instance, X+Y=2, X and Y can be called both 变量 and 变数.

Although, 变数 has more meanings than 变量 - as you mentioned - it's usually used metaphorically. It means variables or something irregular or unexpected.

i.e. 人生的道路上充满了变数；定好的计划却出现了变数.

--Grace