That's because its genetic composition is 75% plum and 25% apricot. A good friend of mine in Santa Cruz has apriums growing in his yard, which are also a plum and apricot hybrid, but with the reverse composition. They are a bit sweeter than pluots. I have been told however that there are some aprium varieties that are not as sweet as some pluot varieties. I use pluots frequently in my jams and have found the late-season ones available in Northern CA to be both very tart but also to have a low pectin content.

It is a cross of plum and apricot, more like a plum though.