Why is 十面埋伏 related to the number 10? How does that count?

As we know, Chinese directions can be categorized as 六合八方, but none of them is related to the number 10?
KEY defines:
十面
as:
(on) all sides, (from) all sides

Also from a Baidu Zhidao question:
十面指的东,南,西,北，东北，东南，西北，西南,上，下 

Ten sides: east, south, west, north, north-east, south-east, north-west, south-west, above, below