I don't know if it was intended or not, I actually like it, but the index value of 1.0 is actually "just before" the first frame interpolated with the last frame and not actually the very last frame of the sub-sequence. The only reason I bring this up is that the documentation does not mention this nuance and it may be nice to have it included in there, honestly I think the nuance is important for smoother animations anyway.
I noticed it by trying to write a non-cycling animation sub sequence driver class, it looked like it would cycle even though it was ending on the last point. Took me a lot of annoying back checking and cross referencing to realize what was really going on. Now the challenge of figuring out a way to accomplish a non-cycling animation efficiently, I love challenges though.