It's because it's whether it outpowers the haubyberk over time, and 1.5% is 3 extra hits out of every 100 (ignoring the fact double attack may kill some of those) which depending on x hit build is half a WS. Not gamebreakingly huge, certainly, but the game is designed around changing many smaller things to bring out a bigger difference.
Oh, and apologies to you, Massimo. I misread.
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