Hey Mark,

I am not much of a blues player but I do believe what you are seeing is simply an optional variation.

To explain:

In your first example, there's one bar of I chord, then a TURNAROUND bar of V chord. It gives it more momentum.

The second example (from Power Chords as riffs) just stays on I for two bars at the end, which is a bit less interesting!

There are slightly different things you can do with the 12-bar blues, so that's what's happening here. There are additional endings and turnarounds and variations that can be thrown in, depending on the song or situation or players.

I am sure someone with more specific blues knowledge can give you the correct term.

Here is the video referenced, by the way, for anyone else that is curious:

https://www.guitartricks.com/lesson.php?input=14427&s_id=936

-Carl.