You don’t say what files these digital signatures relate.

If it is an installation file only (setup_av_free.exe, etc.) then the virus definitions inside would be periodically updated or you could be installing the latest avast version but a very old. So the act of updating the contents would require the digital signature to be updated also.

I have an old version of the installation file for 6.0.1289 and that was digitally signed on the 6th September 2011. Hover over the installation file and it will tell you what the program versions.