Dogs don't generalize well.  
This is why you have to teach them to do something, then practice with them doing that something in a million different situations to get them to be really and truly reliable.  You saying 'come' to your dog when you're in the house or on a walk or whatever looks and feels different to the dog than when you leave them on a sit-stay, walk away leading with your right leg, turn around, stand tall, look them in the eyes and say 'come' in an obedience ring.  I'd guarantee the quality of your voice is different in this situation than it is in casual situations as well. 
Use the same word, use different words, it really doesn't matter, the situation is different enough that the dog will behave differently in a practicing or competing in formal obedience situation than it does in a casual situation.  What will matter to performance in the ring is that you do fronts and finishes in your front yard, in your back yard, out on the sidewalk, at the park, at training class, in the pedestrian mall, etc, etc, etc.
In terms of having a dog that always comes when called regardless of distractions, (independent of the issue of whether it just comes close to you or does a front), an argument has been made that you should use classical conditioning linked to a word that the dog is never going to hear under any other circumstances so as not to weaken the power of the conditioned response to that word.  This is a theoretically sound approach, although I reckon most people would fail in doing the sheer amount of work to accomplish it.
Here is a link to Shirley Chong's instructions on how to carry out this conditioning:
http://www.shirleychong.com/keepers/Lesson6.html
Good luck and enjoy training your dog!