午 is noon because it doesn’t have a “horn” sticking out like cow 牛
矢 is arrow because there’s a straight arrow across the top unlike fault 失
平 is flat because it’s flat across the top unlike 半 which indicates 2 halves
I’ve got all of these pairs wrong in the past many times before I started using the above
Similarly for 曲 and 由. The former has more vertical “bar lines” like a sheet of music. Reason has a single vertical line ie one reason.
Hope this helps.