Can’t donate blood for a year

I went in to donate blood today, and turns out when I last donated (nov 2024) my initial hemoglobin from the finger prick was fine (14.2) but then they tested my vials and my hemoglobin was below 8. The guy told me the scale goes to 8 and mine was lower. I was supposed to receive a letter telling me this, but I never did. Im really confused how this can be. I’m wondering if anyone else has experienced this, or if it’s common? How can my hemoglobin half during one donation?

This was my third donation, and my hemoglobin was 15.6 the first time, then 13.9 the second time. I don’t have any signs of low iron and eat the same as I have for a while. Any insight would be helpful!! Thanks :)