I believe the letters are just used as placeholders. In this way a question could potentially stay relevant for decades. Of course in accounting we know that for many questions this will not be the case as standards and tax rates and accounting practice often change radically. Hope this helps.
Yes. It is a sort of laziness shown by publishers so that questions do not have to be updated as years pass. X and Y are eternal; 2020 and 2021 are not.