Ruby array slicing - weird behavior

If you play around array slicing in irb, it will behavior like below:

 

irb(main):027:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):028:0> a[2,1]      
=> [3]
irb(main):029:0> a[4,1]      
=> nil
irb(main):030:0> a[3,1]
=> []

The weird behavior is that sometimes it return empty array, and sometimes it return nil instead, why that? According to the ruby docs an array split should return nil if the start index is out of range. So why is it that a[3,1] is not returning nil?

 

Below is the reasonable explaination from StackOverflow: 

The end of the array is [], but the end of the array plus 1 is nil. Look at it this way: the Array [1,2,3] can be considered to be constructed from cons cells as such: (1, (2, (3, ())), or 1:2:3:[]. The 3rd index (4th item) is then clearly [].