ADG 715 - MUX with surprisingly low OFF resistance.

I think I'm just about to learn a bit of a lesson :(

I've designed a mux to select a low level (<1V) for input to a USB connected oscilloscope using the ADG715 as the selector-switch.

I assumed that when off, the resistance across each switch would be higher than the 5kOhm that I'm reading on my multi-meter.

With the scope input resistance about 10 MOhm, the variation between 5 and 5k Ohm across the switch makes little difference to the measured voltage.

Am I missing something? The data sheet doesn't specify an off resistance, just leakage currents which are in the nA range which lead me to think that the off resistance would be in the MOhm range.

Help ....