It is not clear how the text is used.
Is it to show the number of census records in the Gedcom or is it to inform that new census data is added to the Gedcom or even to the Research Assistant?
You must be looking at the SVN version. This variable does not exist in the CVS version.
In the CVS version, the text "census added" is hard-coded in two places in "blocks/block_htmlplus.php". It provides narrative for the first and last events of the GEDCOM.
In other words, when the very earliest or the very latest event of the GEDCOM is "CENS", this text comes into play.
I can't tell you why the word "added" is there. It doesn't add any meaning to the text.
Yes, it's in the SVN version, in the includes/class_stats.php file. The reason it says "census added" is it's ment to describe the event (birth/death/census added).