I have tested my website 2-3 times for performance testing and each time I got different results. I do not understand why as url is same and page size is also same?
The response to a test will vary depending on the network conditions, the load on the server, the processes running on the browser. That is the primary reason WPT allows you to execute multiple test runs and then reports on the median result.
To get a reasonable measure of performance, run an odd number of tests at different times in a day. From this, extract the average / median to get a decent idea on the performance number for the day.
Hope this helps.