Your experience isn't uncommon - many find they shoot more accurately with a wider rear notch or narrower front sight.
Often, a stock sight/notch setup leaves little daylight on each side of the sight, so more daylight makes it easier to tell if they're equal.
Also, as the widths of daylight get greater, they get closer to the width of the front sight, and it's easier to to align 3 equal-width objects than it is to line up 2 that surround an oddball 3rd.
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