In the original Epworth data, patients with insomnia scored lowest of all seven groups
Murray Johns's 1991 sample of 180 adults put patients with insomnia at an average Epworth score of 2.2, below the 5.9 of healthy controls. Being tired and being sleepy are different measurements.
Group averages from the paper that introduced the Epworth Sleepiness Scale (Johns, Sleep, 1991): healthy controls, 5.9. Primary snorers, 6.5. Sleep apnea, 11.7. Narcolepsy, 17.5. Insomnia, 2.2.
The lowest score in the sample belonged to 18 patients whose complaint was that they could not sleep. Their range was 0 to 6, and the group scored significantly lower (p < 0.01) than every group except the healthy controls. The gap against healthy controls came close to significance without reaching it (p = 0.063).
The reason is in the question. The scale does not ask how tired you feel or how badly you slept. It asks how likely you are to doze off in eight situations, such as sitting through a meeting or lying down in the afternoon when circumstances permit. In that last one, 94 percent of controls reported some chance of dozing, while the insomnia patients reported either none or only a slight chance. Johns read this as low sleep propensity, even when able to relax, and added that it should not be assumed for every kind of insomnia, mood disorders being one he named.
So the nightly sufferer and the person who nods off at the cinema can sit at opposite ends of a sleepiness scale while sharing a complaint of fatigue. A low score tells you the second problem is absent, not that the first is.
The group was patients with persistent psychophysiological or idiopathic insomnia, which is narrower than insomnia in general, and 18 people is a small sample. Feeling worn down also has causes unrelated to sleep propensity. Where the Epworth sits among other scales is laid out in a comparison of three of them.