Note that there are 'different' types of sensors. Most sensors you see monitor the temperature close to a certain component (cpu, northbridge, ..) while other sensors measure the temperature inside a chip through a thermal diode inside the chip.
In most cases the cpu temperature is measured using an external sensor and the gpu temperature is temperature inside the gpu. Some gpu sensor chips also show the board/ambient temperature which is the temperature close to the gpu but not inside. The difference between the gpu inside or near the gpu is roughly 15C.