To be precise, Kprobe does not measure or report "unreadable sectors". In other words, the fact that it cannot read them does not mean they are unreadable. Kprobe locks the drive at one read speed (mostly), and just skips over sectors that can't be read at that speed. It does not allow the drive to re-read or slow down, as it would normally do. There are other reasons that cause it to skip, one that the author pointed out is PUH slippage.
It's best to view the read errors as a flag that you may have a disc that needs to be scanned slower, or that it has other issues.
You can also sometimes see these read errors occur sparcely on a perfectly good disc when scanned at high speed. This is one reason why I often choose 40x as a scanning speed, it seems to eliminate a lot of these things.