Why 7805c Voltage regulator drops the current if the current enters around 13mA to 7mA?
Have a look at a typical datasheet and see the data related to the "quiescent current". That is the current that flows from the input out of the GND pin (the input current is the sum of the output current and the GND current). 



The maximum GND current is 8.5mA over temperature (provided output current does not exceed 1A) but the guaranteed specifications don't tell us much about the typical behavior. So turn to the datasheet curves, and you'll find these: 




Now you can see the typical ~5mA added to the output current and since the current varies slightly with the input voltage you can also explain the 100Hz (double-mains-frequency) ripple in the input current, whilst the output current remains quite constant (since it's a voltage regulator feeding into a fixed resistor). 



"Quiescent current" is really a bit of a misnomer in this case- the GND current should hardly be considered "quiescent" when the output is supplying 1A, but that is the customary name for the current used internally by the regulator. Note that it includes the current used by the regulator proper plus the current used by the internal voltage divider on the output to divide the output voltage down to the internal reference voltage (R17/R18 in the schematic). The value of R17 is not shown because it is different for different regulator output voltages (eg. 7805 vs. 7812).