It probably is the angle you are viewing them at. When it is in the primary bucket, you are viewing down at an angle. When viewing the test tube, you are probably just about straight on. The test tube would be the more accurate reading, andread the lower part of the liquid, not where it curves up on the side. If you take a couple test readings and view from the same angles every time, you can use that(let's say 1 point) difference to make a correction.
By the way welcome to the forum. I hope you stay a while and share your experiences.