It seems that 100% makes the textbox a little wider than the space for the image. You can also see, that the round corners are cut off on the right side. So the textbox “hangs over” to the right a little bit.
Width is set to 96% because featured text area has padding of 20px on each side making it larger than those 100%. Removing padding makes text to appear right on the edges of the box and it won’t look good.
I would recommend using percents or max-width because with your current setup you are losing responsiveness. Of course unless you change @media properties.