Stem Variants

I don't want this post to appear spammy, so please forgive the way I explain this.

From my signature you will see what my targetted words are. The thing I've noticed this morning that surprised me, is that in Google the results are different depending on how you put the words into the search box. For example, if you seperate them with a hyphen, do both words in lower case, put the first letters in capitals or capitalise the whole words, each SERPS result is slightly different.

I was under the impression search engines used "stem variants" so you didn't have to bother about such things. Has anyone else noticed this and should I worry about it?