Yes, it is. They are measuring the milk in isolation, which in this instance means to pour (and measure with your eyes) as it fills the empty container before the tea. Whereas you can't really measure how much the volume is changing (eyeballing) in isolation.
Think of it as the difference between measuring a liquid in a container and eyeballing a liquid in another liquid.
> measure a specific amount of milk in isolation than to eyeball how much of it you've just added to something else