If you use the normal Google.com page and search for your targeted keywords, you run the risk of artificially increasing your impressions. Since you aren't likely to click on your ads while conducting the search, you're also going to be lowering your clickthrough rate with each search. This can hurt your quality score and your ad positioning. It may cost you more to maintain your current ad positions. So, you really need to be using the Google AdWords Ad Preview Tool instead of Google.com to check your ad performance for certain keywords.