Difference between List<String>.from() and as List<String> in Dart

Question

I have read a snapshot from Firebase, and am trying to extract the value from a node that is a list of strings.

When I do this:

List<String> answers = snapshot.value["answers"] as List<String>;

With the above I get a runtime error saying:

type 'List<dynamic>' is not a subtype of type 'List<String>' in type cast

But somehow these two approaches below both work:

List<String> answers = List<String>.from(snapshot.value["answers"])

Or this:

List<String> answers = snapshot.value["answers"].cast<String>()

What is the difference between the first and the other two constructs, and why can't I cast the List<dynamic> to a List<String> with the as casting operation?