Smarthawk
You are indeed correct but it is getting an accurate picture to see how the engine is performing and that is difficult over a short period, for instance if you have a pilot who insists on topping an O-360 up to 8 USQ at each flight you are likely to be looking at an oil consumption of 0.75 USQ/HR. This on an engine that if kept topped up between 6-7 USQ is likely to return a consumption of 0.1 USQ / HR.
Unlike aircraft operated by airlines we don't have the same control over the operators and so one pilot who disregards the oil top up policy of a flying club can disproportionately affect the consumption numbers over a short period of time, the result of this is the engineering staff making judgments based on unsound data.