Back in 2003, Google introduced something known as "Expanded Broad Matching" to their Adwords advertisers. This Expanded Broad Match essentially replaced standard Adwords broad matching, and brought a whole new batch of search terms into play for keywords within advertiser accounts.....
Traditionally, back when things were simple and transparent in PPC, a search term had to include all the keywords within your Adwords keyphrase in order to trigger your broad match keyword. So, if you had the keyphrase "big red shoes" in your account, it would only be triggered if someone typed in a search phrase that included all three of these words. If they types in "buy big red shoes" your ad would show; if they typed in "red shoes" it wouldn't. Simple. However, somewhere along the line, the waters got well and truly muddied by Google.
Nowadays, your ads can show for just about any search phrase that is in any way related to your keywords. Sometimes, Google cutting you this kind of slack and flexibility is good. Take the example above. You would want your ad to show up if someone typed in "large red shoes", rather than "big red shoes", as they really mean the same thing. So this type of broad matching is good. However, Google have taken the idea of what is "relevant" too far.
I've seen some absolutely crazy examples of broad matching in my time. For example, the keyword "plane tickets" getting triggered by the search term "train schedules", the keyword "dyslexia" being triggered by "diarrohea", and even the keyword "kangol hats" being triggered by "san diego zoo".
Some have a very cynical view of expanded broad matching, feeling it's simply a way for Google to show more ads for more search terms, and therefore make more money. My grumpy side agrees with this. However, the happier, Friday afternoon side, thinks that the algorithm for matching keywords to search terms just isn't quite right. I think Google is trying to be helpful here, but has just gone slightly overboard with its matching formula.
They need to reign things in. At present, our ads are showing for too many irrelevant searches. Even if they don't get clicks, the low CTR is harming our quality score and effectively costing us money through higher CPCs. So, Google, can we please go back to just "Broad Match" and leave the expanded stuff to the lazy advertisers? Or, at the very least, allow us to opt out of the expanded broad match and defer instead to simple broad match.
In either case, something needs to change. If they don't, we'll spend our whole lives running Search Query Reports and adding negative keywords......