Compare Number of Rows of Tables
A subquery with UNION ALL and the aggregate function avg() used as window aggregate function should do the trick:

SELECT tbl::text, ct, round(ct - avg(ct) OVER (), 2) AS deviation
FROM  (
   SELECT tableoid::regclass AS tbl, count(*) AS ct FROM t GROUP BY 1
   UNION ALL
   SELECT tableoid, count(*) FROM tbl GROUP BY 1
   -- UNION ALL ...
   ) sub


SQL Fiddle.