Number of tuples (20300) not the same as heap (20301)

I have a table in which the first four variables are the unique primary
key. Recently, I ran a vacuum and was told that the number of tuples did
not equal the number in the heap for the primary key index to this
table. I'm not exactly sure whatn this means. It sounds like one or more
of the "unique" keys may have more than one set of data.

I often update the table using a text file containing the "UPDATE
table_name SET ..." and running the \i command in psql to execute the
commands from the text file. For some of these updates, I update an
variable length array that sometimes is longer than 8K. So what I do is
just update the first 100 or so positions in that array and get the
remaining positions with another update. I'm wondering if somehow there
is a bug that doesn't like this a ocassionally screws up the indexing.

Anyone know what this error means? I've tried dropping the index and
re-creating it but still get the same error on a vacuum.

Thanks.
-Tony