tzdb often has the answer, and it's almost always because everyone used to do local solar time (i.e. on average, the sun is at the top of the sky at noon) until

tzdb often has the answer, and it's almost always because everyone used to do local solar time (i.e. on average, the sun is at the top of the sky at noon) until convenience (i.e. train and plane schedules) required an offset friendlier to mental math.