Exists() basically checks on identical values in the symbol table so it will only filter your input table if you have loaded the exact same values before.
I assume your table with the 12 months are basically showing dates, i.e. timestamps only for midnight (AKA integer values).
Your timestamps may show any time of the day? Then you are effectively only filtering the timestamps that are recorded exactely around midnight.
Just for clarification, your timestamp field may be formatted as date, but still show numeric values with a time part.
Please have a look at