Search Key Word vs. Search Key Phrase

For a number of years now I've noticed that many of my pages do not do well in a Google search when a one word phrase is used. I've never been able to figure out why my pages do better with two or more key words rather than one key word, but I guess I can't show up first for every key word.

So one example is the search term VME, which would be a one-keyword search. The VME page shows up in the ninth position on the first page. When the phrase is changed to VME Bus, the page shows up in the forth position [same for VMEbus]. When I add another key phrase, VME Bus Pinout, the page shows up as the first listing on the first page. First listing for the key-words VME Backplane, and VME Chassis also. Hmm, looks like VME Connector places first too, in both a text and image search.

I'm using the VME Bus as an example, but I could have used any computer bus because the same thing happens. I just don't under stand how Google determines what page should show up first when only one key-word is used.

Yes I know the rules, those other pages must be using the key terms more often at the beginning of their article. I' don't know, my pages do show up but I would rather have them show up higher for a few one word searches. I guess I need to somehow optimize the VME interface page up that it shows better in a Google search, maybe rewrite some of the text.