I don’t understand, why does it have to be flush side to side?

https://www.decorpad.com/photo.htm?photoId=173552

Edit:

https://cabinetsolutions.com/cabinets-bathroom

Picture 21 is a solution. You can see that the sides under the counter have little wedges to make the counter fit the space above the cabinet. I personally don’t see the point but if it’s the look you’re going for, it works