Any part surfaced on a properly trammed machine should exhibit “suck-back”. If you place the surfaced face on top of a reliably flat, hard surface, then try to lift it up or peel it away, you should feel some resistance, like the part is stuck to the surface. This “suck back” only happens when there is very little space for air between the two surfaces so they act as if there is a vacuum between them. Hence the pressure of the surrounding air tries to keep them in contact. Basically the same principle as a vacuum fixture.
The larger the contact surface area, the flatter the surfaces, the higher the suck-back force (and therefore force required to pull them apart).