Margrave is only given to the Duke which could be any lord in the duchy.
I'm not sure if you are just improperly expressing yourself, but one does not need to be a margrave to be a duke. However, in most title systems, the only regions from which a ruler can create a duchy are the ones held by margraves.
Also, there is no set method of determining who gets what titles. The ruler picks a title system. In the republican system, for example, every lord is a senator. This has nothing to do with government system. In IVF, a theocracy, every lord was called senator.
