How one keyword matches thousands of unrelated searches
Broad match is Google's default matching type, and it's far broader than most advertisers realize. Add the keyword 'running shoes' to a broad match campaign, and it will bid on 'shoes', 'running', 'athletic footwear', 'jogging gear', 'best tennis shoes', and thousands more. Google uses its semantic understanding to expand your keyword into conceptually related searches, even if you never wrote those variations. A single keyword might match 2,000 to 10,000+ different search queries depending on the category and seasonality.
The expansion tree grows taller the more popular your base keyword is. 'Running shoes' for an athletic company might reach dozens of subtopics: shoes for marathons, trail running, road running, indoor training, high arches, flat feet, mens, womens, kids. Each branch subdivides further. Google isn't random about this expansion, though. It uses signals like your landing pages, your ad text, your account history, and real-time auction data to predict which queries are most likely to convert.
Controlling waste through negatives and landing page structure
The tradeoff is volume versus waste. Broad match captures demand you didn't know existed, which is valuable early. But it also clicks on tens of thousands of queries that have nothing to do with your product. If you bid on 'running' broadly, you'll lose money on people searching for 'running water', 'running backs', 'running start'. You can't add every wrong variation as a negative keyword fast enough.
The solution is layering. Use broad match for high-intent, high-volume keywords where the expansion is tight and add negative keywords aggressively. Separate your landing pages by product line and route each campaign to its own page, so Google's matching algorithm has clearer signals about what you actually sell. Use phrase match for secondary campaigns to capture some expansion volume without the chaos. Monitor search terms reports weekly and negative out the patterns you see. Broad match works best when you build guardrails.