Jane Simpson, a freelance writer who writes frequently about pets, wrote a recent article about a dog's capacity to understand commands. The article is entitled, "Training Your Dogs: A few things you must consider".

A point she makes is that dog's don't necessarily hear the specific consonants in a command, the but the inflections of the voice:

A dog responds to like sounding commands having different meanings but a similar inflection of the voice. For example, a dog that has been trained to the command "Heel," if ordered "meal," "reel, "steal" or "veal," will obey as readily, despite the fact that he has been told to do something entirely different or that he has heard an expression devoid of any meaning at all as a command word.

I've encountered a similar situation, when I reward my dog Max with a "good boy", he's actually hearing only the "boy". Because when I've said "bad boy" he seems to wag his tail and get happy.