You are correct, in general.
An engine with a knackered camshaft, for example, will need more fuel for a given power output or, alternatively, will not make rated power and will thus fall short of book takeoff performance, during "full power" takeoff.
Also you sometimes get a different prop fitted, to the one which the POH is based on
The only way to be sure of fuel flow for a given IAS is to do some tests. Do two flights which are identical except for the length of the stabilised cruise segment, and start and end each one with a full tank. Then you can work it out. Or have a fuel totaliser installed