The suggestions bar tries to offer a useful set of follow-up operations based upon the type of a result. In the case at hand, the result is a list of lists. But it so happens that all of the sublists are the same length, which happens to be the representation of a matrix in Mathematica. So the suggestions bar has guessed that the user has a matrix in mind and has offered follow-up operations that make sense for matrices.
However, it also realizes that it may have guessed in error. The user might be thinking about a simple list that, by sheer coincidence, has sublists of equal length. Hence the offer to treat the result as a list of lists instead. If we accept that offer, we are shown operations that make sense for nested lists:
... along with the option to switch back to the matrix operations we were shown in the first place.
The suggestions bar does quite a bit of this inferencing. The goal is to read the user's mind -- but to give back-up options in case the mind-reading facility picks up the wrong signals ;)