Why doesn't the A320 pack flow indicate HI as it should?
From FCOM §Air Conditioning - Controls and Indicators:


  The pack flow indication can be up to 30% below the actual flow rate.


That's why the needle may not be at the HI position. LO to HI is 80-120%, with normal being 100%.


The aforementioned needle (YouTube).

The sensor responsible for sending this data is the delta P (differential pressure) sensor at the inlet of the FCV (flow control valve). My guess is that it is not designed for a precise reading, but only an indicative reading.