Mri shows three lobulations in bone above knee. Bone scan says everything ok. How could mri and bone scan show two different results?

Different studies. The meaning of "lobulations" in the bone on an MRI study is uncertain. This may mean an osteophyte, cortical thickening, old trauma, a normal variant, or something else. The bone scan measures bone chemistry and activity such as repair of the bone. When the bone scan is normal with these findings on MRI, it means normal bone chemistry, and indicates a none serious problem.
"Lobulations" is not medical diagnosis or useful information with regard to your MRI results. Please consult with your doctor regarding how your imaging results may (or may not) be relevant to your knee problems.