Why does the contrast change when you zoom out of an image on Preview on mac?

If you click on an image file in finder on the mac and press the space bar, Preview will preview the photo for you. After doing this, you can zoom in on the image (by pinching two fingers apart from each other). When you do this it's like a gray layer with low opacity appears over the image (as if there is less contrast). When you zoom out all the way, and continue doing the zoom out motion with your fingers it's as if this gray layer disappears (and the resulting image often looks better.)

If I open the image in another photo editing app, the image with the apparent gray layer is what opens. Thus, it looks like what I perceive as a gray layer is in fact just the image itself. Thus, I'm guessing preview applies some sort of filter when you zoom out all the way of an image and continue zooming. Does anyone know what kind of changes this filter is making (because most of my images look a lot better and have much better contrast with this apparent filter)?

I searched Google now because I also want to know. It's almost like when you zoom out more contrast appears and as soon as you lift your fingers from the touchpad an soft/grey overlay get's applied...