When ancestry doesn't tell you how something has been "updated", it usually means that they didn't add anything, they just redid some of it because they had made a mess of it the first time around! No kidding. For example, everyone knows how awful the census indexes were. In this case, "updated" means they had some volunteers go through and redo the indexing. After all, census is census. There will never be any more of the 1880 census, it's over and done, and was complete the first time around. It was just the index that was horrific. So now it is called "updated", LOL.
Another example is the Civil War pension index. For many years, ancestry claimed, on their database explanation, that 10% of the cards were "missing". The truth is that they scanned those cards directly from the NARA microfilm, and none of them were "missing" from the microfilm (I use that microfilm all the time at NARA). Cards that were very dark or very light on the microfilm did not transfer over because the people who did the images the first time didn't know how to use the scanning equipment, apparently, or didn't want to take the time to adjust it properly for each image. Thus many images were not readable, and if an image was not readable, they just chucked it and pretended that it didn't exist at all, and didn't even index the name. There were actually about 30% of the cards missing from ancestry's database, not 10% as they claimed. After many years of me and others complaining to NARA about this (since they gave a partner agreement with ancestry), ancestry has FINALLY redone that index.
Now you know why they don't tell you what has been "updated"--it would be too embarrassing!