This was recently written by Ancestry.com and featured in Dick Eastman's online Newsletter:
"Today we’ve launched enhancements for six U.S. Census collections – in addition to the improvements on six censuses released a few months ago. In all, we’ve gone through more than 200 million records to improve images and many indexes.
"Enhanced and clearer images are now available for the 1790-1900 censuses, and indexes have been improved for the 1850, 1860, 1870, and 1900 censuses. All of the images have a cleaner, crisper look. And occasionally names can be read that were illegible before—either because they were too light, too dark, too blurry, too faded, or covered in tape, etc."
If you haven't looked at the census lately, perhaps you should revisit them.