Ads that appear in favorable ad positions on the first search results page receive more impressions than those that appear on subsequent results pages. So, it makes sense to get your ads into those favorable positions. For more info on ad position, see What is "ad position"?
Using bid adjustments can make your final monthly spend greater than the maximum estimated spend. However, if a customer who does not match the target criteria clicks your ad, you do not pay any incremental bids.