MS Project, Resource Availability, Task Usage work hours

Question

I've been using other tools for awhile and have to jump back into MS Project.

I started a high level plan with tasks and resources, however, I'm having trouble remembering the specifics around Task Usage work hours, resource availability and resource assignment.

In my resource sheet, I create a resource:

Resource Name = John 
Type = Work

In Resource Information, I set the following for John:

Available From  Available To    Units
NA              9/30/2020        0%
10/1/2020      11/30/2020       20%
12/1/2020      12/31/2020       40%

I then created a task:

Name:   Task1
Start:  10/1/2020
Finish: 12/31/2020
    Start no earlier than 10/1/2020
    Fixed Duration

I then assign resource "John" to task "Task1".

I then look at Task Usage with Timescale set to Months, and see the following:

            OCT      NOV    DEC
Task1
   John     35.2h   33.6h   36.8h

Because I set availability to 20% for first 2 months and then 40% for month 3, I would have thought that I'd see:

Month 1 = 23 days x 8 hours x .2 = 22 x 8 x .2 = 35.2 
Month 2 = 23 days x 8 hours x .2 = 21 x 8 x .2 = 33.6 
Month 3 = 23 days x 8 hours x .2 = 23 x 8 x .4 = 73.6

But, Month 3 shows 36.8 (23 x 8 x .2)

Switching Timescale to weeks confirms that each week is showing 8 hours, even those in December.

I know duration, unit and work are related but can't see what I'm missing.

Why doesn't December show 73.6 hours?