8 Coal cooks 63 items, rather than the usual 64 [Server]

I put 64 Cobblestone and 8 Coal in a furnace on a server. When I came back a while later to pick up my Stone, I found that only 63 had cooked, when I was certain that coal was meant to cook 8 items. I first shift-clicked the coal into the furnace, and then I shift-clicked the Cobblestone.