Originally Posted by RonnieG

Depending on your overall daily budget, when you have "Standard" ad distribution set, your ad may not show up every time a user's search terms match your keywords. Think of it this way: If there are 900 user queries per day matching your keywords, and your CTR is averaging 10%, that would be 90 clicks, with an average of say $1.20 per click. That would be over $100/day. So, if your daily budget is only $25/day, Google knows your average CTR and tries to even out your ads so they will run evenly throughout the day instead of spending your budget in the first 6-8 hours. That means your ad will be shown only 25% of the time that it could have. The option of having your ads displayed for every matching query could mean your budget would be spent early, and your ads would not be displayed after that happens.