TransferFunctionModel Question

Here are three ways to define a transfer function model. From the documentation, it seems to me that all three should produce the same thing. But the third case, which specifies {nunerator, denominator} = {a, b s} seems to result in something different. What is it I am misunderstanding?

Here’s the documentation I’m reading:

If you look at the notes under the section, Details and Options, it says you have to specify num as a matrix… den can be specified as a matrix or just a denominator.

The following works as expected:

In[1]:= TransferFunctionModel[{{{a}}, {{b s}}}, s]

Capture.PNG

Yes! Thank you very much, David.
(This community is wonderfully useful.)