Guys, thanks for your replies.
bb: Like gmanjesh mentioned, they basically provide answer explanations to questions in the OGs in much greater detail than those that are there in the OGs.
gmanjesh: Thanks for providing the link. The feedback in the thread is quite helpful.
bonedry: Yes, that is correct. Siewe's book hasn't yet been updated to cover OG 12
but then again, it's for $29.99, compared to GMAT Hacks' $69.99.