列表下标问题

carcar

list[[1]] <- list(c("a","b","c"),c("x","y","z"))运行结果：
[[1]]
[1] "a" "b" "c"
[[2]]
[1] "x" "y" "z"


我如果再扩充list，以下两种方式：
（1）list[[1]][1] <- c("X","Y","Z")
【运行结果】
Warning message:
In list2002[[3]][3] <- c("X", "Y", "Z") :
  number of items to replace is not a multiple of replacement length
（2）list[[1]][[1]] <- c("X","Y","Z")
【运行结果】
> list2002[[3]][[3]]
[1] "X" "Y" "Z"

为什么第二种是对的呢？[]和[[]]有什么区别呢？