That is due to the height of the sights over the bore- at close range it will shoot noticeably low.
I always zero with Federal XM193… if I’m going to shoot something else and I’m not being real serious about it, I’ll just shoot a few rounds of the other ammunition and see where it impacts. I’f it’s close I’ll just make a mental note of it, if its drastically different then I’ll change the zero making a note (on paper) of what it took to zero so I can walk it back to the XM193 zero. My Nikon has been very consistent at returning to zero.