One other slight difference (between boot-time and "normal" on-demand scans) I'd questioned a week or two ago and Vlk was kind enough to confirm.
If you've set up exclusions (folders which are nothing but JPG's, for example), a boot-time thorough local-disk scan will ignore your exclusions and scan everything. That's apparently because when running the boot scan, avast doesn't yet have access to the setup info where you've defined the exclusions.
Or at least that's the case in Home, and it's a reasonable guess that the same is true in Pro also.