People often use a different symbol to denote the same meaning.
For example, an author used symbol a for a data sequence. However, the same author, in another article, uses the symbol x to denote the same data sequence in the same context.
Why doesn’t he use symbol a for the data sequence in the second article? Is it just to make the manuscript look different compared to their previous one? Note that there is enough degree-of-freedom to use the same set (or sub-set) of previous symbols in next manuscript.
Nevertheless, it becomes more complicated for the readers when they use Greek symbols together with superscript and subscript. I agree that people have their own choice to use the symbols in their manuscript, however, why would the same group of authors use a different symbol for the same meaning in another manuscript?