AIM Section 7 Chapter 2

"Michael 182" wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...
Was flying in the Denver area today and approach told a plane that FL 180
was unavailable die to low altimeter, which was 28.87 at the time. What is
the reason for this? Confusion with 29.92 readings?

Thanks,

Michael