A backup would be a filesystem level event, so at best it would only access 1/N of the sectors (where N is the duplication level). It isn't guaranteed to access all of the sectors the data resides in -- and also, it may have hit the kernel's filesystem cache or the on-disk cache or something like that, too, bypassing the physical media entirely.
Also keep in mind that a RAID rebuild is a media level event, it doesn't just copy files. It will copy over the "empty" sectors, too, because it just has no idea what's what on the disk. It faithfully recreates the raw bytes on the media.
The disk might have failed in a part of the disk you aren't actually using and never noticed because all of your actual data is intact -- the the RAID can't really rebuild without them.
A backup would be a filesystem level event, so at best it would only access 1/N of the sectors (where N is the duplication level). It isn't guaranteed to access all of the sectors the data resides in -- and also, it may have hit the kernel's filesystem cache or the on-disk cache or something like that, too, bypassing the physical media entirely.
Also keep in mind that a RAID rebuild is a media level event, it doesn't just copy files. It will copy over the "empty" sectors, too, because it just has no idea what's what on the disk. It faithfully recreates the raw bytes on the media.
The disk might have failed in a part of the disk you aren't actually using and never noticed because all of your actual data is intact -- the the RAID can't really rebuild without them.