Depends on how many and who was doing the proof reading. It also depends on what they were using. I have to go over my posts several times just in here, before I hit the post button to make sure auto correct hasn't changed what I typed, and believe me, it does sometimes. I've watched it change what I actually typed to something else right before my eyes, and that's with my original still being spelled correctly, so it wasn't a spelling error correction. If you're typing fast and not paying attention to what's showing on the screen, the software can be changing things without you realizing it, especially if it also has syntax correction.
Also, the last person who should be doing proof reading, especially final proofing is the person who wrote it. Their brain knows what they meant, and will correct an error in their mind because it knows what's supposed to be there. Final should be done by someone who hasn't read any of it before. Depending on how many people they have in that department, they may not be running the drafts by enough people.