By the Puranic period the Adityas are twelve, one presiding over each month of the year, and Parjanya is where the two standard lists come apart. The arrangement preserved in the Vishnu Purana names him among the twelve and gives him a month and the fixed company of sage, gandharva, apsaras, yaksha, serpent and rakshasa that each Aditya travels with. The list in the Bhagavata Purana also has twelve names, and Parjanya is not one of them; Savitr stands where the other text puts him. Ten of the twelve names are common to both and four are not, and Parjanya is on the disputed side of that division.
The atlas records the membership as claimed rather than settled, which is the only honest way to hold it. The tradition did not reconcile the two lists, and there is no third text with the standing to decide between them. A reader who meets Parjanya among the twelve has met the Vishnu Purana arrangement, and a reader who does not find him there is not holding a defective copy.
There is a plausible reason for a rain god to be in the class at all, and it is the same reason a compiler might drop him. The twelve Adityas of the Puranas are a solar group matched to the solar year, while the older Rigvedic Adityas were not solar but sovereign, concerned with oaths, shares and the right ordering of things. Parjanya fits neither description exactly. He is a weather god inside a group that had been about law and became about the sun, and an editor working from either end of that development had grounds to leave him out. What the disagreement preserves is the seam between the two ideas of what an Aditya is.