Ask HN: Why is the Mac Copy/Paste experience so inconsistent?

When I command-shift-4, I expect a copy of that portion of the screenshot to go to the clipboard. And then I expect to paste what I copied. However, half the time it properly copies, and I can paste the image correctly. The other half of the time, when I paste, I see the previously copied item. Then, I see the screenshot appear in the lower right, and then it appears on my desktop, and I have to either a) re-copy and re-paste or b) drag the desktop image to my destination. Am I not doing the right method? Note: I'm a Mac user for >15 years.