R的do.call函数具有意外的意外行为

Question

当与do.call函数组合时，我遇到了R mean函数的意外行为。

do.call(max, list(2,6,4))
6 # as expected 
do.call(min, list(2,6,4))
2 # as expected
do.call(sum, list(2,6,4))
12 # as expected
do.call(mean, list(2,6,4))
2 # should be 4!

我在这里想念什么？