因为一词多义。

balance:
n. 平衡；余额；匀称

vt. 使平衡；结算；使相称

vi. 保持平衡；相称；抵销

I found it difficult to balancework and family.

我发现要保持工作与家庭的平衡是很难的。

She had trouble keeping her balance.

她保持平衡有困难。

Athletes need a good sense of balance.

运动员要有良好的平衡感。

He lost his balance and fell backwards.

他失去了平衡，往后跌倒了。

He lost his balance and fell to the ground.

他失去平衡摔倒在地上。

At that moment he lost his balance and fell.

他在那一瞬间失去平衡摔倒了。