Well it's from Wiki, but it corresponds from what I've read:

It was insufficient, however. In 245, the satrap of Parthia, a man named Andragoras, revolted from the young Seleucid king Seleucus II, who had just succeeded to the throne. In the confusion, the Parni attacked and seized the northern part of Parthia, a district known as Astavene, probably in 238. About 235, a Parnian prince with the name Tiridates (Modern Persian Tirdad, meaning 'Great archer') ventured further south and seized the rest of Parthia. A counter-offensive by king Seleucus ended in disaster, and Hyrcania was also subdued by the Parni.

From then on, the Parni were known as Parthians. In the years that followed, their kings recognized the Seleucid king as their superiors, but under Mithradates I (171-138) they conquered Media, Babylonia, and Elam from the Seleucids. The Parthian empire was to last until AD 224, when it was succeeded by the Sassanid empire.
So Parthia the province succededed, then the Parni came in and took it over. It seems that the Parni were not in actual Selecuid lands, or at least lands that they had any control over.