NIW-I140 for additional EVIDENCE

My NIW-I140 has been REFERRED for additional EVIDENCE. It says:

"If applicable, please evidence in the form of letters from current or former employers indicating that you have at least 10 years of full-time experience in your occupation. "

I have worked as Postdoc for a couple of years, and some years of half-time RA before my Ph.D graduation. Why do they ask for at least 10 year full-time experience evidence? Would it be helpful to collect all employment letters in my resubmission? Thanks much if you could provide some advice and help.