Your projector is not square to the wall.
1. TURN OFF ALL DIGITAL KEYSTONE CORRECTION - NEVER USE IT!
2. If the right side is smaller, then turn your projector towards the side which is smaller. It will make it bigger.
3. Once both left and right sides are the same size, you want to check that the top and bottom are the same sizes.
4. If the top or bottom is smaller, then tilt the projector up/down until the top and bottom edges are the same size.
5. Do not worry about getting it 'centered on screen' - that is what optical lens shift will take care of.
6. If everything appears to be square, but the image is lightliy rotated, then adjust the yaw on your mount so that the top and bottom edges of the image are in line with the screen. This is only possible to get perfect if the image is square so....
7. Repeat steps 2-6 until the image is as square as possible.
8. If the image is not on the screen itself perfectly, adjust the optical lens shift, zoom, and focus to reposition the image until it is on screen perfectly. You may find that it is good to make little changes to zoom and lens shift during steps 2-6 so that you have a working guideline.
I generally make the image slightly smaller than the screen size, then align the top edge of the image to the top edge of the screen then look for a non-square image. If I end up putting the image several inches off the right (or left) side of the screen, I use lens shift to get it back on the screen, then I check again. Ooops! It's smaller at the top then the bottom, so I tilt the projector up some, then lens shift it a bit so it is back on screen and check again. Raised it to much! Tilt the projector back down a bit. Use lens shift to raise the image a bit. Good tops and bottoms, but the left side is still a bit smaller... Turn the projector to the left, and lens shift to the right. Okay, it looks square, but the top edge of the projected image doesn't line up with the top edge of the screen perfectly. The right corner is a bit low. Adjust the yaw of the mount so the right side of the projector is raised a bit. Dang! Now I'm readjusting everything to get the image square. The entire process takes about 30 minutes to an hour the FIRST time you do it and you see how much movement creates an impact on alignment. But, after you are comfortable with it, about 15 minutes and you get a very solidly square image on screen.
NOTE: It is possible, that if you have a cheap screen, the screen itself is not square. This is not common and isn't indicated by the picture you have here.
The image you showed indicates that the projector needs some yaw adjustment to raise the right corner and it needs to be rotated to the right as well then lens shifted to the left some.
