The first stage of the check involves the clerk rattling off the particulars on the 4473 to an operator, who is basically a data-entry clerk. The clerk gets one of three possible results: proceed, denied, or further review.
The first two are self-explanatory. "Further review" means that the matter gets kicked over to another operator, who has more comprehensive access to records.
For one reason or another, some combination of your personal data kicks up a "check this further" flag. It could be due to any number of factors.
In the depth of winter I finally learned that there was in me an invincible summer.