They won’t be able to tell you anyway. Just wait for GCMS notes.
Try not to be too alarmed as review required is relatively common around here. I’ve seen reports of people getting negative remarks on their notes (like employment address seems to be residential, cannot verify employment etc) but it turned out fine.
Does your employment letter state exactly what they need? Hours, duration, address, salary etc? Are you employed by a large or small firm? If it’s missing information that could be why.
And does it match the NOC code description? Even “job descriptions too similar to NOC website” is a reason for review required. But in the end it’ll turn out fine.
Are you employed by a large or small firm? Often, larger firms can seem more legit and be less subject to scrutiny. If it’s a small private company that’s barely found on the internet it’ll take longer to verify.