From past experience, when the percentage figure goes back it could be an indication that malware has been found. If any malware is found, avast goes into paranoid mode (for want of a better word), this requires a recalculation
Of course it could be something else.
The first few scans may take longer as it builds up the persistent cache of known good files (typically those with digital signatures). That said you need to have the scans options set to gather that data.
The smart scan runs several different scans (depending on installed components), so for the easier scans it could generate higher percentage figures.
Although what Eddy said about the known bug, a fix was meant to have been issued. Whilst it is extremely possible you haven’t received that micro update yet, try rebooting and keep the system up for at least 15 minutes and repeat the process again. That would hopefully pull the micro update in.
