According to Wikipedia it's supposed to be York County concessions.
Though given that Queen was southern edge of the first concession and the second; Bloor was the southern edge of the second conession; Eglinton was the southern edge of the fourth concession ... clearly it's not related to the old York Township numbering.
It would appear that 16th Avenue is between lots 15 and 16, and this is surely the origin of the name ... but why is the road between lots 5 and lot 6 called 14th Avenue - a retronym perhaps?
The the lot numbers along Yonge are somewhat different ... perhaps 16th and 14th predate the 1800s townships ... though that would make Steeles 13, Finch 12, Sheppard 11, Lawrence 10, Eglinton 9, St. Clair 8 Bloor 7, Queen 6 ... and that seems pointless ...