The problem is the difference in length of the hydraulic circuits going to the two cylinders. The tubing going to the left cylinder is longer with more bends so it reacts slower due to greater resistance to flow. Hydraulic fluid, like electricity, is "lazy". It looks for the easiest path and takes it first. The left cylinder dosen't catch up until the right cylinder is stroked out.
The bucket acts as a tiebar between the two cylinders. Without the bucket the cylinders are in the same position fully retracted and fully extended but rarely in between. I run without my bucket quite a bit and your situation is common.
