I’ve been thinking about the different ways fae develop from their parent creature - you could shake fairy dust over six identical sibling mice at the same time, and see six different mice-fae result. Perhaps wildly different, there is no stricture on the faeriscite agent to be reasonable.
In our world, Darwinian evolution is a two part process; Darwin rolls the dice, then the environment weeds out as many individuals as possible; the successful individuals survive to reproduce, making better dice for Darwin to roll up the next generation. After a generation or two, basic body types emerge to fit environmental niches.
The fae of the Wood are liberated from Darwin's environmental weeding phase. Each one is unique, and if it survives long enough to scatter fairy dust over a potential new host, the faeriscite has been successful. No basic fae body types need to emerge - the fae do not breed, they need not worry about mating ritual or threat display or the pressures of the predator-prey relationship.
What the faeriscite needs, however, does have its effect on its children. The faeriscite, to reproduce, needs its host to age and release spores in a place with many potential new hosts. For the faeriscite, Darwinian pressure pushes it to create fae who will be tolerated by creatures who might otherwise wish to avoid the contagion of fairy dust. Therefore, the fae, while wildly diverse in body type, do tend to have certain features in common; clever hands, sharp eyes, an appealing appearance (at least to the dominant culture in their area), and always, a powerful, burning desire to help.
And who can resist that?