The second decan of Libra runs from 10 to 20 degrees of the sign and is ruled, in the scheme this atlas records, by Saturn.
The card on the decan
In the Golden Dawn's assignment this decan carries the Three of Swords. The mapping is one to one and exhaustive: the thirty-six small cards numbered two to ten across the four suits match the thirty-six decans exactly, taken in order from the Two of Wands at the first decan of Aries. The suit follows the sign's element, so Libra is a Air sign and takes a card of Swords, and the rank follows the position in the sequence, which puts the three here.
Three layers, three periods
The decans are Egyptian: thirty-six star groups whose successive risings told the hours of the night, attested on coffin lids from the Middle Kingdom onward. The division of the zodiac into thirty-six ten degree segments with planetary rulers is Hellenistic, several centuries later, and it is a different object that inherited the name and the number. The tarot assignment is later again, and belongs to the Hermetic Order of the Golden Dawn in the late nineteenth century. None of it is Egyptian, and this atlas records the three as three.
What the atlas is recording
The degrees and the ruler are the attributions this atlas holds for this decan, and the card is the Golden Dawn's. All of them are traditions with variants rather than settled facts, and the decan itself, as a ten degree segment of a sign, is the only part that is simply arithmetic.