The TH hair could have come from 1.5% of the population. The Jacoby hair could have come from 7% of the population. However, whoever is the source of the hair must share a maternal ancestor with TH and Jacoby, respectively, and must have been in West Memphis, Arkansas on May 5, 1993. Yes, it's circumstantial, but so was every scrap of evidence (except Jessie's stories) against Damien, Jason and Jessie. Those percentages, as I understand it, represent the percent of the population with a common maternal ancestor as determined by mtDNA.
Let's put the shoe on the other foot. Suppose that a hair was found at the discovery site that was linked to 1.5% of the population, including Damien and another hair was found that was linked to 7% of the population, including Jason. Would a non dismiss those hairs as the TH and Jacoby hairs are being dismissed? Early on in the case, there was a hair (or hairs) that was said to be "microscopically similar" to Damien (which has since been found to be false). That comparison doesn't even come close to this potential match! However, it was presented as evidence against him, and the jury, out of ignorance or fear or whatever, bought it! Even TH has stated that the hair is "probably" his, but of course he claims innocent transfer - which, as has been explained by others, doesn't really hold water on an active eight-year-old boy who he claims he didn't see all that day.
Occam's Razor says that the simpler of two explanations is usually correct. What's simpler? (1)Three teens, in a drunken state, kill three eight-year-old boys whom they did not know and do so without leaving any physical evidence. (2)An adult (or adults) kills three eight-year-old boys, one of whom is (one of) the adult's step son, move the bodies to hide any evidence but inadvertently leave a hair at the discovery site. My money is on #2!