Why is the power consumption of the filter lower/higher than what is indicated on the packaging ?
Unfortunately, we must assume that the value displayed on your ammeter is incorrect. There are numerous ammeters which do not include the blank value (cosinus phi) in measurements correctly.
You can only determine the actual output of a filter with a corrected device or by multiplying the voltage times the current (amperes) that is measured:
V * A * cos Phi = W.
However, it is advisable to gauge the flow rate by litres for operation in an aquarium. The external filters have the following flow rates in a cleaned condition and with a hose length of 1.5 m:
CristalProfi e700: 350-400 l/h
CristalProfi e900: 380-450 l/h
CristalProfi e1500: 800-900 l/h