Why is the mobility of lipids in liposomes greater than that of lipids in biological membranes?

The thickness of biological membranes (5 to 8 nm, measured by electron microscopy) is about that expected for a lipid bilayer 3 nm thick with proteins protruding on each side. X-ray diffraction by membranes shows the distribution of electron density expected for a bilayer structure. Liposomes (lipid vesicles) formed in the laboratory show the same relative impermeability to polar solutes as is seen in biological membranes (although the latter are permeable to solutes for which they have specific transporters). In short, all evidence indicates that biological membranes are constructed of lipid bilayers. https://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Lipid_Bilayer