Confusing Coolant

[garisyaris]

Hi all
Feel a bit dumb asking this.
(see pics)
I would expect when removing radiator cap when cool, as with other vehicles, the coolant level to be at the top, right under cap. However with my 2012 Yaris, there is always an empty space below the cap, which can filled up with about 50ml of coolant.
Run the car, come back coolant in expansion tank a bit higher as expected. Let sit for 8 hours to cool, coolant level in expansion tank a little lower as expected. All working good. No issues.
Take radiator cap off, and there's that small empty space again under the cap. Is this normal? Is it because the cap is higher than the radiator?
An irritating ocurrance, as I can't understand the reason for this, being a dumb bum.
Any explanation appreciated.
Cheers ... GarisYaris