That is statistics, not pure maths. But yeah, statisticians abuse a lot of notations like that, just like physicists. I haven't seen pure mathematicians make such unclear notation.
(Statistics is as much maths as theoretical physics is, both are technically mathematics but in practice the field is handled in a very different manner since they are applied and intended to solve a specific set of real world problems and hence not pure)
Probability theory is not math? (But sure, the same formula can be written in a more explicit way using subscripts and distinguishing random variables from their values.)
(Statistics is as much maths as theoretical physics is, both are technically mathematics but in practice the field is handled in a very different manner since they are applied and intended to solve a specific set of real world problems and hence not pure)