The zodiacal placement of the seventy-two rests on one piece of arithmetic. The circle has three hundred and sixty degrees. Divided by seventy-two, that is exactly five degrees each. Every angel therefore holds a five-degree segment, and the seventy-two cover the whole ecliptic without remainder.
The fit is what makes the system attractive, and it is worth being clear that the fit is a coincidence of numbers rather than evidence of design. Seventy-two divides three hundred and sixty because both are highly composite; nothing about the Exodus derivation anticipates the zodiac.
The segments interlock with an older scheme. The decans are thirty-six divisions of ten degrees each, three to a sign, and they are genuinely ancient, Egyptian in origin, used for time-keeping by the rising of stars, absorbed into Hellenistic astrology and treated systematically by the second century. Each decan splits into two of the five-degree segments, so the seventy-two sit inside the thirty-six as halves.
Where the sub-division was first applied to the Shem angels is not something the primary sources make clear. What is clear is that once it was in place, everything else could be hung on it: the planetary ruler of each decan in the Chaldean order, the tarot minor arcana in the Golden Dawn arrangement, and, through the pairing associated with Thomas Rudd, a spirit of the Goetia for every angel.
Two things follow for a reader. The zodiacal attribution is not part of the derivation and is not implied by it. And the neatness of the whole edifice is a consequence of seventy-two being a convenient number, not a confirmation that the system was discovered rather than assembled.