Using AVG version 6 as an example...
There are two styles of updates out there and AVG has used both. The old way was to download all virus defs and possibly program changes and replace entire files. If you did the manual update, the bin file also contained all program changes so an old copy of the install program and the latest bin file were all you needed to get an up-to-date AV.
The new way is to have a differential file which adds virus defs to the existing files. From the behavior I've seen so far, the differential file is all virus defs since a certain date (update #516). If you skip a file, you still get the virus defs in the next differential file. However, when you're talking about a month or even a year of virus defs, it is still a lot smaller than the full virus def file. At some point, they will make another starting point for the differential file (e.g. when there are program changes as you will have a large download anyway).
If they used an incremental file system (only the virus defs since the last update), then you would need to apply all update files in order to keep in sync. I don't think that is happening with AVG.