Oooh, so close...
Look at where you're measuring, right by a wall: That's going to reflect the sound and give a false reading. Also the tape is going to rattle around as well if it's touching the exhaust, possibly making excess noises as well (quieter than the exhaust certainly, but of a different frequency that it could make a difference). Go and park in the road or in an open industrial estate of something, hold the sound meter and tape measure in your hand and try again. Unless you're silly tall or silly short, then that will be close enough to 45deg and is how every single trackday company I've ever been with does it too.