It sounds like you are watering down the test wort, but not by a lot. Even so, the smaller overall sample will result in a wider effect. The bigger and longer the tubing, the more volume, of course. As to wort reading, I have always allowed the sample to settle out for the reading with the hydrometer to avoid the solids from affecting the reading. Maybe the effect will be minimal, but I don't know how much that would be and a few points are always a possibility, depending on how thick the particulate is.
I know this doesn't fully answer your question and is based on my own personal experience, but it is what I think happened here. Personally, I don't sweat gravity readings anymore (but I brew very low ABV beers for the most part.)