Think of a point where arguing is still possible — there is room around it, and reasons can be put on both sides. That one picture can be read in two directions. Stand on one side and it means nothing here is settled. Stand on the other and it means there is a real case to be made. The word keeps both readings, and the shape of the sentence tells you which one is in play.
From the word’s history
Wiktionary gives arguable as argue plus -able, through Old French arguer from Latin arguō 'to make clear, to prove'. So the word is built as 'able to be argued', which is exactly the open space the modern uses divide between them.
Wiktionary 記載 arguable 由 argue 加上 -able 組成，argue 經古法文 arguer 來自拉丁文 arguō（弄清楚、證明）。所以這個字的組成就是「吵得起來的、論證得起來的」。今天的兩個用法，正好是把這塊空間分成兩邊各佔一半。
Wiktionary etymology tree supplied in evidence packet (kaikki:3f6f69a190036ae4:915115906:14547)
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