When you click 8 and 9, your chart might appear to show the lowest value (8) instead of the Avg (8.5) because of rounding. Check what is on the Number properties, or try adding your expression to a text object so you can check the output.
Your gauge chart should not have any dimensions and should have one expression.
As for how avg() works:
1. Avg() will add the values and divide by the number of values, so (8+8+8+9)/4.
2. If you want (8+9)/2 you would have to do a sum(distinct Value)/count(distinct Value)