Why would an author use a different symbol for the same meaning? 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?
I can't speak about the specific case you have in mind since I'm not a mind-reader, but there are legitimate reasons to change a symbol.


Maybe you want to avoid clash with notation used for something else in your second article.
Maybe that "something else" was introduced in an earlier revision and then eliminated, but then you decide to stick with the new notation.
Different symbols are standard, or natural, for different objects. You can write x(f), where x a function and f is a variable, but people don't, and for a good reason. Most of the time, you give it some thought when you introduce a new symbol, and try to pick the 'best' letter (and the best position for a subscript/superscript).
Sometimes, you realize only after writing an article that a different symbol would be more natural for a certain quantity. Possibly because it's related to something else in a different field, or because you find out that another author used the same symbol for this object in an earlier paper and several colleagues are already used to it. Or because two objects that you use in your paper are related, so it would be better to call them A and Ã rather than A and B.
Sometimes, you write papers targeting different journals/communities, and they have different standards, so you use different letters for the benefit of your readers. For instance, I may use Ax = b if I am writing about a least-squares problem in a numerical linear-algebra journal, or wX = y if I am writing in a statistics/machine-learning one.
Sometimes, it's a referee who tells you that you should really change your notation, and you comply. This might happen on the second paper but not on the first.