The different sensitivity has nothing to do with which viruses/trojans/malware it scans for as it uses the same virus signatures for all scans. The difference in Quick, Standard and Thorough sensitivity is the files that it scans, some files are more prone to infection, like .exe, .dll, etc so all of those must be scanned. The Standard scan will look at more and the paranoid Thorough scan will scan all files, as you have found there is a huge difference in the types of sensitivity used across the scans, if you then add archives to the equation it gets even longer.
Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned. Thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
I have only ever done a through scan with archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.
So there you have it in a nutshell.
The rootkit scan is incorporated in the Standard and Thorough scans but not the Quick scan. A rootkit scan is also carried out two minutes after your desktop is active on normal boots.
