I dunno... I/O errors don't go away.
Under any circumstances, though, Disk Utility doesn't touch every block on a drive, only the ones with which it must deal to accomplish its task (but I can't guess why it may have touched those items in one pass and not another).
I
very strongly urge you to run this command (I remembered the process being more complicated,)
immediately, rather than rely on DU's results!
Copy and paste this into a Terminal window:
sudo dd if=/dev/rdisk0 of=/dev/null conv=noerror
Hit return, enter your password at the prompt (it will not be visible), and be prepared to wait "n" hours for the command to run its course, depending on how fast your MBP runs, how large your HDD is, and whether it runs at 5,400 or 7,200RPM.
The command runs in the background, so you don't have to quit working while it's doing its thing, and
I again urge you to not be lazy! If the original I/O errors can be relied on (Edit: And there's absolutely no reason to believe they were spurious.), your HDD is liable to tank at any moment...most likely the worst one.
Good luck!
Edit: Please take a screenshot of your completed Terminal session and post it so we can see precisely what's up with your MBP.
