Question

why do blood, tissue fluid and lymph differ in what they contain

Answer 1

The answer you are looking for depends on what parameters of the three fluids you are asking about. For example, blood and extracellular fluid (ECF) differ because of blood pressure and the presence of albumin in the blood: the pressure inside the blood vessels tends to make fluid move across the vessels and into the ECF. This is particularly the case if blood pressure is high or atmospheric pressure is low, leading to excess fluid in the tissues (look up edema if you are interested in physiology of altitude). Normally, the presence of albumin tends to draw fluid back across the blood vessels by osmosis.

This would be one mechanism by which blood and ECF are different but another might be that blood is well oxygenated and contains nutrients whereas the lymph has less oxygen and nutrients after the cells have absorbed them. The lymph will have more waste products of metabolism from the cells too. Once the lymph has passed through a lymph node then there will be more white blood cells in it - these are called Lymphocytes. I hope all this gives you an idea of the processes involved.