COUNT_TABLE always returns the number of rows in that table (FILTERs are respected).
When you use a COUNT on a column of a table, it will not count the NULL values in that column. Therefore, COUNT will return a smaller result than COUNT_TABLE in that case.
Another reason for different results might be that if you use COUNT in an OLAP table with queries that operate on other tables (the Activity table, for example), the COUNT is done based on the join of all tables involved. This way, you might get much larger values than you expect. COUNT_TABLE however always returns the number of rows of the given table, independent of other dimensions.