Try it sometime! (actually don't!) When you put a vacuum hose up to the spark plug hole you are actually creating a greater negative air pressure inside the cylinder (especially with the valves closed), thus sucking whatever dust,dirt etc around the spark plug hole right into the cylinder. Air filter? That does provide a slight restriction, so air is gonna come from path of least resistance.

When you take a air gun with a longer nozzle and put it in the plug hole and blow compressed air in the cylinder the pressurized air excapes the plug hole and blows out the dust/dirt away from the plug hole.

Same thing goes for when you pull axle shafts and you want to get the sand/dirt out of the axle tube. Use a vacuum cleaner and you'll pull all that stuff right into your differential. I guarantee it.

-J