This would just be the normal response of our kidneys to when we don't drink water enough (aka just normal).
Proximal tubule is isotonic because we are reabsorbing BOTH Na+ and H2O.
Macula densa is hypotonic because it is sensing the distal convoluted tubule -- the most DILUTE part of the nephron. Remember the countercurrent exchange system - thick ascending limb is losing NaCl (urine gets less concentrated as it ascends).
Medullary collecting duct is hypertonic because we are reabsorbing all the water making the urine more concentrated.