Difference in flow measurements in an open channel

I’m undertaking a civil hydraulics question.

It involves the flow of water in an open channel down a river at $5m^3/s$ until the flow is split in two by a weir acting as a sluice gate allowing flow into another stream. A weir pool is located on the upstream side of the gate. $2m^3/s$ flows into a stream not dictated by said weir and I assume we apply conservation laws so that $3m^3/s$ flows into the stream under the gate. It’s stated that the gate is set to a certain height and the weir pool height is a function of the flow rate into the other stream.

The initial question pertained to finding the height of the gate above the river bed, however, the following question states that subsequent flow measurements were taken and indicate that when the gate is set at the calculated height, the flow into the stream under the weir is actually 10% less than what is calculated (I assume $2.7m^3/s$). It then asks, why could that be the case?

My first thoughts were that this could be due to frictional effects from the gate and possibly across the river bed, maybe a change in the channel geometry or possibly a change in slope. Have I covered all the possible options here or is there more that could be affecting the flow rate in this situation?

Any help would be greatly appreciated.

Cheers