[SecondCar]

I checked windshield washer fluid reservoir and it looked low so I added fluid and it filled up the spout but didn't appear to fill the reservoir. Is this an optical illusion or could there be something at the bottom of the spout blocking the fluid from getting into the reservoir?

 

[maybeanotherone]

Mark the level on the fill tube and run the washer a few times.

 

[SecondCar]

Thank you for that suggestion which I did today. The level in the tube went down, which was puzzling since the white space at the top of the tank remained the same. So I pulled off the spout at the tank and discovered a plastic shield all around the filler hole inside the tank and about an 1" in height. This shield is preventing the tank from filling up completely due to the trapped air, and I'm guessing it's there to prevent the tank from rupturing when full and it's below 32F. "Problem" resolved.

This picture shows the amount of unused space in the tank with the filler full.

 

[wth]

The OP's pic of the reservoir filled up into the neck looks just like mine. I think you will find that the filler neck/tube extends down below the top of the reservoir an inch to allow expansion and allow extra room if water was just in the reservoir and froze.

I was going to post yesterday but ran out of time. Then I posted just now, then read the OP's reply, and I see he's one step ahead of me.