80 stardust!?!

I got it once too. Caught a ditto and i noticed its only 80 stardust. I then thought ditto gives 80 stardust but subsequently caught a few ditto and they gave at least 100 stardust each. Could be a bug

It's because the Pokemon it was disguised as received a weather boost where Ditto did not. Example being if it was a Gastly in cloudy weather the Gastly would receive a stardust boost, but since Ditto doesn't get a boost in cloudy weather they need to cheat it so Ditto gives out the amount of dust it normally would. It's not a bug, rather a curious workaround.