the last jump phenomenon

It was brought home to me, the other day, how big the gap can be between what I think I'm saying, and what someone else interprets my words to mean. This is a common occurrence in everyday life, for example:
" You look nice in that dress" , a simple statement, which says what it means. Change that to " You look nice in that dress, the bigger size is a better fit". Now does it mean you look nice, or that you're fat?!!
Two statements in one sentence, which can be interpreted in entirely different ways!
That's a crude analogy I know, but I use it to illustrate my point. People don't always hear what you want to say, and neither do our dogs. 
Sometimes you think you are showing your dog something very simple, but the dog interprets it the opposite way because to them there is another message there.
A simple example: The dog does a lovely course, but stops before the last jump and looks round at you, hesitates, or even takes it from the backside. You are yelling "go, go, go" , why doesn't he take the jump?
Well , you may have slowed down as you get to the end , hence saying in fact "we're about to stop now". Or you maybe dropped your arm as you approached, and he then thinks " Oh, maybe I'm not supposed to jump after all"