Checking competition in Google - diffrent results with proxies?

I'm developing a tool to check competition numbers in Google, among other things. I noticed a strange thing though. Because the tool makes a lot of requests to Google, I need to use proxies to not to get banned. But the the numbers for the EXACTLY the same url and keyword differ when using proxies from different countries , eg:
-proxy IP from US: keyword "iphone" - 433,000,000 results
-proxy IP from Germany: same kwds - 367,000,000 results
-proxy IP from Portugal - again - 378,000,000 results

and so on...

That's pretty big difference...and sometimes numbers differ even when using different proxy, but from the same country too...

Why is that so? Do you get that with other kw research software too?

Yea i've seen plenty of this. Market Samurai will show keywords having 0 searches per day, but google keyword tool will give that keyword at least like 500 per month or something... i typically grab the data from MS and throw it into a worksheet with the data from G's tool, so i can look at it from different angles.