Exploring England and Wales Census Statistics

The bar graph shows official census totals in blue and the total entries in the corresponding Ancestry database in red. Six of the eight have more entries in the Ancestry database.

I made the compilation as I've occasionally calculated derived statistics based on the total number of entries in the Ancestry database assuming they were close to the official census totals. How well to they reflect the true population?
The number in the Ancestry collection could be larger because of duplicates, alternate transcriptions. Ancestry might have a smaller number where the original enumerator sheets have been lost and only the statistics from the census survive. Findmypast has a list of missing census returns at http://www.findmypast.com/articles/census-for-england-wales-and-scotland-missing-pieces

The differences are less than 1.5% except for 1841, 1851 and 1901 where Ancestry has 7.2%, 8.6% and 1.7% more entries than for the official census.