Every one of the five term rulers holds exactly one term in each of the twelve signs, so Mercury's stake in the Egyptian table is twelve stretches of unequal width, and this 8-degree term in Aries is one of them. Added together those twelve come to 76 degrees of the zodiac's 360, the third largest share any of the five planets holds.
The whole division runs Venus 82, Jupiter 79, Mercury 76, Mars 66 and Saturn 57, and the five figures sum to exactly 360 with nothing left over. That ordering belongs to the table itself rather than to any modern gloss on it: the two planets Hellenistic astrology called benefic hold the most ground, the two it called malefic hold the least, and Mercury sits between them. The same five figures circulate in the Hellenistic sources as the planets' years of the terms, so the arithmetic that divides the zodiac and the arithmetic that measures out a span of years are the same arithmetic.
Measured against its own twelve, this is among the widest terms Mercury holds anywhere in the table, matched at 8 degrees by its terms in Libra and Scorpio, while at the other end it keeps only the 3 degrees it holds in Pisces. A ruler's share is therefore not spread evenly around the zodiac, and reading a chart by the bounds means reading a different width of ground in every sign.
What the surviving sources do not supply is the reason for any of it. The table reaches us as a table: the order of the rulers and the size of each stretch are transmitted without the principle that fixed them, and no reconstruction has settled the question. The atlas records the figures as they are transmitted and keeps the question of their rationale open rather than supplying an explanation the sources do not carry.