Why do astronomers say that the sun rotates once every 27 to 31 days, rather than give an exact number?

Astronomers cannot give an exact answer about how long it takes the sun to rotate because of the sun's matter. The sun is made up of gas and plasma, these types of matter have different weight than the solids that most planets are made out of. Different parts on the sun move at different speeds giving a range of days for the sun's rotation.