Thank you, OP. As was explained to me in the Ordnance Officer Basic Course, sometime in the 15 or 1600s the English used one (ordinance) to address construction of roads, bridges, buildings, etc; and the other (ordnance) specifically for military equipment (firelocks, cannon, edged weapons).
At least that's what they told us. Maybe people back then were, like today, just shitty spellers.