Venturi Duct flow

When water flows through the converging side of a Venturi, why is it that the total head (experimental and calculated) is not constant? Could it be due to losses?

Also, why was it that a lot more total head was lost when the fluid came out of the diverging side? Could it be due to turbulent flow?

And why does volume flow rate effect steadiness of flow?

Also why is the total head different for the experimental and calculated data as soon as it enters the Venturi too?

This is my data, sorry it doesn’t let me upload a picture as it’s too big:
Axial position: 0, 60.28, 68.68, 73.18, 81.08, 141.54
Static head: 230, 194, 158, 121, 75, 130
Total head measured: 235, 230, 230, 228, 224, 149
Total head calculated: 234, 239, 244, 249, 242, 134

I don’t get why the total head please starts at 1mm difference and how it remains the same at 230mm for the experimental one and how the calculated total head is increasing?