Why would an author use a different symbol for the same meaning?
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.