In a ladder attenuator the internal switches are used
to select a set of (only) 2 resistors. These two resistors
form together a voltage divider. The attenuation is
set by the (relative) value of these resistors to each
other. Each position selects a different set of resistors.
total series resistance of each two resistor set is
equal, so the signal source always sees the same impedance.
The advantage of a ladder attenuators is that the signal
only sees/travels through two resistors.