What causes deviation between theoretical and actual discharge in a venturi?

Construction of venturi meter?

The following are the main parts and areas of venture meter:The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.After the entry, there is a converging conical section with an included angle of 19' to 23'.Following the converging section, there is a cylindrical section with minimum area called as the throat.After the throat, there is a diverging conical section with an included angle of 5' to 15'.Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venture meter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge, etc) as shown in diagram.