Like most built in / on board sensors, take the SMART temps with a pinch of salt.
I have two WD drives in a rack in front of a slow moving 120mm intake fan and was concerned that one was reading 12 / 13 degrees higher than the other.
I used an external thermal probe to check the temps at three locations on the HDD sides they are actually within half a degree of each other.
For perspective, current readings with a 28 degree room temp:
WD1200JD 38 degrees in SMART, 34.9 degrees on the probe
WD2000JS 52 degrees in SMART, 34.4 degrees on the probe