Why my "Copy picture of selected area to the clipboard" does not work as usual? macOSX 10.13.6

I am frustrated with screenshot capture shortcut on my MacOS X.  It becomes a camera icon and it only captures the window rather than area selection, like the following picture. 



I did not change anything, and I have no idea why it suddenly works like this.  The original function should look like this: 



Appreciate any help.
You can switch between area/window screenshot with a spacebar. Here's the Apple website about the screenshots.