Using third party tool like pingdom, gmetrix and even Google page speed it is obvious to get variation in result. There are many factors on that page speed depends.
For example: If broser cache is enabled on site then very first check with any of this service will take longer time and after other consecutive tries it may load faster as the page is already cached.
With my own site http://saur.in (under development)
First attempt on pingdom: 1.83s
Second attempt: 786ms
That is almost haft in time and more than double the page speed. The same way server cache plays its role too.
There are other factors like domain name resolve, dns connectivity current traffic on your site and many more.. so result may vary. While working with performance we have to consider average response time or page load time.
At last, one more thing, I should add here:
Create a set of results from 2-3 services and check performance after every 2-3 hours. Pingdom provides testing for every minute and its free... you can try that. Try if for 24 hrs and look average result of it.