i didn't do nothing,

i didn't do nothing,
美剧里,i didn't do nothing,经常被翻译成我什么也没有做.
可是一般我们不都是用i did nothing,或者,i didnt do anything 来表达这个含义么?
按书本说的双重否定等于肯定,那么i didn't do nothing不就成了我做了一些事了么?
经常发现美剧里喜欢这样的句式表达否定,但一直不懂为什么,