Has nothing to do with key cap material or printing. It's due to how the case resonates with the sound of the dome collapsing and the air being forced out of it. The rubber dampens the sound of the plastic impacting, and the escaping air gives it a hollow-ish sound. The case then resonates and adds its own character. A plate will dampen that resonation somewhat. The larger the plate, the larger the low frequency dampening effect. That's why the HHKB thock is much deeper than the Realforce, and only slightly deeper than the 660C.