reltuples value less than rows in the table.

I have a index in a table. The value of the reltuples value in the pg_class
table for this index is less than the number of rows in the table where the
index is present.
For eg. if i have 800 rows in the table , the reltuples in the pg_class for
the index show the value as 769.
What are the scenarios under which this kind of behaviour occurs?
Also iam not able to execute the queries involving the indexed column in the
where clause.
Please suggest the probable cause for this.
Also if i do a reindex on the table, the reltuple values updates to the
current no. of rows ie. 800.

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