The liquid left in the hops due to squeezing rather than weighing/drying/weighing is definitely a source of error, but I would wager is not tremendous. The volume measurement is from the bottom of the kettle, after wringing out the hops and subtracting the volume left by my dip tube (~8 oz.)...
Thought I might offer an update on the original post: You can still get all of the parts listed/shown at Lowes but whirlpool has gone to 3/8" fittings on the filter housing instead of what I think was 3/4" when johnsma22 put it together in 2007. The blue plastic line that comes with the filter...
I collected the hops left in my kettle after the wort had gone to the fermenter and wrung them out by hand and got 20 oz. of liquid out of 2 oz. of hops.