This!
Iron needs Vitamin C. In college, I had to eat my burgers with a glass of orange juice. Can't be taken with antacids or calcium.
Most people forget that most vitamin levels depend on a secondary vitamin already in place like vitamin D and B6 when taking magnesium but it can't be taken with calcium.
So it isn't just that one vitamin is low. It often means that other vitamins are also low, just not in the dangerously low levels that would get attention. But depending on diet, age and how you are taking in Fe, it can change how much iron is actually being absorbed by the body.
That is why most modern vitamins with also have listed "other" vitamins included. Because they need each other to be absorbed.