It can be a few different things. SEOmoz sums them up pretty well here stating:
"Why do the rankings in SEOmoz not match what I see in Google? Great question! Although there is never one “right” ranking for any keyword, SEOmoz tries to represent the ranking results that “most” users will see. If you see wildly different results from your search engine compared to your Ranking Report, it could be because of the following reasons:
Personalization – Most search engines will personalize your results based on your search history. Make sure you are logged out when performing a search. For Google, try adding &pws=0 to the end of your query string.
For example, if you searched for “SEO” the URL in Google would look like google.com/search?q=SEO. To turn off personalization, append &pws=0 to the end, so it would look like google.com/search?q=SEO&pws=0.
Geographic Bias – Search engines also customize results based on location. This means users in one city often see very different results than users in another location.
Latency – Rankings can change between the time we retrieve them and the day when they are viewed.
“www” vs. non-“www” Subdomains – The Web App is subdomain specific, meaning it only tracks rankings for the specific subdomain you track. So if you enter “example.com” as your subdomain, but “www.subdomain.com” is what ranks in the search results, this ranking won’t appear in your rankings report. This also might indicate a problem with canonicalization or duplicate content.
If you set your campaign up on the wrong subdomain, the only way to correct it is to start a new campaign. You can either archive or delete your old campaign, or choose to keep it running if you have enough campaign slots available."
You can also choose to use a different browser to check your rankings.For instance, I use IE for my main surfing habits, Chrome for my SEO and web development, and Firefox to check my Google rankings. I do this, because I do my Google-ing in IE - so things get cached, Chrome has me signed in all of the time because of my Google accounts, and Firefox I never use.
Hope this helps.
Mike