nizo 801 manual exposure problems

[Osheen Keshishian]

i recently purchased a black body nizo 801. i shot an exposure test and was confused/disappointed with the results. i metered with my sekonic and shot an 18%greycard. an incidet reading at the center of the card read a f.4/5.6 . i bracketed from f22 to 1.8 I figured that there may be about a half stop light loss due to lens and prism veiwing system so f.4 would look right. i rated tri x at 160 and knew not to expect much latitude in either direction: maybe two stops. but it came back darker than i had expected. i thought maybe iris was sticking. looked through the gate and it was moving smoothly throughout the scale although i have to really use some finesse to hit the mark i want, especially between 1.8 and 2.8. then it occurred to me that maybe the indicator doesn't accurately "indicate" whare iris is really at. does that make sense? what am i doing wrong?