Moving from Comment to Answer for better visibility:
To start with, I would clean up your query a bit to ensure it is not something with a misinterpretation of ambiguity. Although the default join is inner join, I would recommend explicitly stating it to ensure understanding of your intention as well as to make it an easy reference when reviewing the code. Also, I would put an alias on EmailAddress in the Select statement (
a.EmailAddress) to again remove ambiguity and reduce risk of misinterpretation.
Outside of that, I would verify if you are in a child account or the parent account. Because if you are in a child account, you should use the prefix
ENT. on the _Subscribers Data View. Making it become
from ENT.[_subscribers] a.
This tells the query to look at the enterprise level (parent) for the data view, which is where the 'All Subscribers' list lives.