Before anyone huffs and puffs, the best I can tell at a glance is because of how Salem's PO games went (blowout, blowout, 1 point win, blowout, 21 point win) they were greatly rewarded and since Louisa and Blacksburg especially were rated highly (Louisa is 6 here, Blacksburg is 9) that caused Salem to be greatly rewarded.
They assign a value to the game, win or lose, based on the rating of the opponent + or - the MOV. In cases where the MOV is less than 15 then 15 is assigned (this prevents Salem beating Blacksburg by 1 being worth barely more than Blacksburg losing to Salem by 1, instead its like Salem won by 15). In cases where the MOV is 30 or more, 29 is used in order to prevent awarding too many points for a poor schedule.
Westfield, the 6A winner who is #4 in this poll, won their PO games by 14, 29, 20, 11, and 7 points thus they had 3 games with the minimum MOV value of 15 assigned and only one with the max of 29 assigned.
Salem won their PO games by 36, 29, 1, 35, and 21 points thus they had 3 games with the maximum MOV value of 29 assigned and only one with the minimum of 15 assigned.
Basically, Salem is #1 because they blew out* 4 of their 5 PO opponents and the computer had several teams ranked very high as well as having Sherando at 31 and GW at 33. There's obviously more to the formula than that but most of the top 10 shared a very similar SOS and opponent W-L record although a few schools stood out there like Salem, HSHS, Westfield, OS.
 * I do NOT consider Salem to have blown out Louisa. However, computers don't watch games thus to them 21 points is considered a very strong win.