Accurate speed is hard to measure. Running at 2 gallons per hour is very due able. Now will that translate into 6kts or 8kts. Lot of that depends on sea state, currents, wind, and how clean the bottom is.
Few people actually lie about fuel consumption. Unfortunately, most are giving best guesses as they don’t have any truly accurate way to measure fuel consumption. When working with numbers like 7 or 7.5 the difference is nearly 10%.