I would say that is fairly normal. The focus of two lights is never going to be 100% the same. The gobo sitting in the wheel slightly different can easily cause this. Also the hot spot being off can cause this as well on the lamp. And if you have the iris closed slightly on one, which might be the case as your question suggests, that would throw it off more.
I find it is more common for the focus to be off slightly on lights with 16 bit focus. Mac 700s, for example, I generally have to adjust individually for custom gobos to come out razor sharp despite being same distance to the target, etc. If I grab all of them at the same time, they line up close but I have to grab each one by themselves and tweak slightly.
Something else to think about is the fact that if it is metal, metal expands and shrinks as it gets hot and cold, this would also have an effect. Glass, being glass, won't be 100% identical. One may have slight flaws or slightly thicker, etc.
If it is way off, then there is something wrong with the fixture. But being slightly off is common I would say. It also depends on how sharp and image you are looking for.
