Surely that is not a true reflection of your rankings, due to personalised search. Different machines will give you different results, which depends on the previous sites you have visited on the computer - how many times you have visited particular sites/how long you stay on the site etc... The main computer that you use to visit your own site will show you ranking higher then you actually do?
I am sure there are some differences, but 1) I generally don't visit my own site! :-) and 2) to a certain extent it does not matter which ranking system you use, as long as you are consistent in using it. I am primarily interested in whether my rankings go up as a result of work I do, stay the same or go down, for that reason I think I need to be consistent in the way I check rankings.