They aren't lieing to you.
The practical performance difference, in gaming, isn't too much - the 280X may have the brute force, but the 380 makes up for it with better tessellation and shading. Gaming aside, the 380 has some other tertiary things, like True Audio and updated decoders.
I think the 380 will support more features from Directx12, but don't quote me on that.
In addition to that the 280x uses more power, but it is slightly faster, has the most minute raw power advantage compared to the 380.