This is an interesting question. I don't know if I'm correct about this, but here are my thoughts.
The differences might be theologically meaningful to a certain extent (e.g. the meaning of diatheke in 2 Cor. 3:14), but they probably have, in my opinion, a more important social function. By using specific language (or jargon) the group can distinguish and separate itself better from other rival groups or even from the rest of the world.
The behaviour of JW's is very often characterised by a radical rejection of the doctrines, practices, festivities, imagery and language of other (christian) religions. So their behaviour might just be the way it is, because they want to be contrary to the world.
In a certain period, JW's did not even consider themselves 'a religion' ('religion is a snare and a racket'). Later on, the language clearly shows they make an effort to separate themselves from christian churches: 'christianity' (JW's, true christianity of the bible) vs. 'christendom' ('professed christianity').
Since there is such a strong desire to be separated from the world/other groups, it is for example not surprising that the symbol of 'christendom', the cross, is rejected by JW's.
Although linguistic and other arguments can be made to sustain the literal meaning of stauros as a 'torture stake', the drive to be different from (other christian) religions may play a (major) part as well. The same can be said in respect of the word 'congregation' instead of 'church' (ekklesia), 'Hebrew scriptures' instead of OT, 'overseer' instead of 'deaken' etc.
Because of the aversion for (other) religious groups, there appears to be even a certain paradox around JW's: they mix very humanistic, secular and scientific thought with biblical arguments to stand out, e.g. their traditional preference of BCE and CE, the insistance on a strict seperation of state and church, their battle for freedom of thought and freedom of press, etc..
Now that the secular world has become more vocal to use BCE and CE, interestingly JW's have recently begun to move away from this terminology.
Most surprisingly, they admit that the surviving texts of the NT have been manipulated thouroughly (because God's name has been removed out of it), but at the same time they still accept the text as exceptionally authorative.
The insistance on a traditional medieval rendering of God's name has also some function (in this way they create the impression they worship a different God with a distinctive name and have a personal relationship with God, while other groups are far removed from him because they do not even know his name and use only titles).
See for interesting articles about the specific language of JW's: