Eugene has replied with a comprehensive answer to this question:
Volumes can show the different date, because -tl works for every volume separately. Only timestamps of files stored in current volume are considered by -tl for that volume.
It is already used for each of volumes, but only files contained in each volume are analyzed for that volume.
If you mean analyzing all files and applying the same latest file time to all volumes, there are technical obstacles.
Firstly, volumes can be created on removable media and users are allowed to remove them after they are created and never insert back.
So we must set time immediately after each volume is created, not when all volumes are created.
Secondly, after we prepared the list of files to archive, some of files can be removed or locked before we actually need to open a file.
Suppose that we detected the latest file time based on the initial file list, assigned it to several volumes on removable media and then, when we need to open the file with latest time stamp, we found out that it cannot be opened.
Volumes are already removed, they have a wrong time stamp of file, which is not contained in our archive, and we cannot fix it, because volumes are already removed. If we apply the latest file time based on entire file set to all volumes, I do not know how to avoid such situation. Asking a user to insert all volumes, one after another, again is annoying for users and we always did our best to avoid it.