When/whenever.

Ok, so my first blog, not surprisingly, deals with a linguistic phenomenon. As my friends know, I am an avid viewer of television :P, and many of the programs I watch are in american English. I have noticed several times lately that some Americans seem not to distinguish between the word "when" and the word "whenever". This morning I was watching Animal Planet. It was about a dog that had suffered gross neglect and was in a poor state when they found her. She was given treatment and fostered by a kind man who, nevertheless, does not seem to master the distinction "when/whenever". This is what he said about the dog in question: "She looks a hundred percent better than she did whenever I first got her". Maybe I wouldn't have thought much of it if I hadn't heard this mistake many, many times before. Or is it a mistake? Has anybody else noticed this? To what would you attribute this phenomenon? And, how would you describe the semantic distinction (the distinction in meaning) between "when" and "whenever"?