1. bad head gasket
2. leackage somewhere in the system
a.some bad hose where the coolant goes through
b.bad water pump
c. add some other reasons

3. Evaporation of the coolant due to temperatures
Whenever i had to add coolant no more than 200-300gr were needed.
Finally i have noticed that the level of the coolant sometimes is different, that is, lower and then at the same place again? Why this is happening?
I am sure that no1 1 and 2 isn't the case for sure, as this is happening some years now.
Any experiences?