Low hemoglobin but iron normal?

I’ve noticed that I’ve had low hemoglobin for some time now. I’m a 37 year old male with 12.9 hemoglobin level but my iron levels are normal. My doctor still wants me to take iron supplements for a couple months to see if it raises my hemoglobin levels. I feel fine besides sorts feeling tired and weak at times. But I work a full time physical job and have 4 kids including a toddler which drains me at times. So it’s hard to tell if I’m tired from that or actual low red blood cells count. What could it be?