Different ranking

Different google servers, google instant search versus not, whether they use shared or private proxies, whether they are caching the results or requesting from google each time. The lower rank you are and the newer your site is for that keywords, the more it will vary each time.

SERP tools are like weight scales. Best thing to do is find one you like and stick with it. If your scale at home says you are 150 and tomorrow it says you are 148, you can be fairly sure you dropped 2 pounds. That is more accurate than using a different scale the next day.

Have you tried our SERP tool listed in our signature? We hit google every time, we also show when we pulled the results for you. We use private proxies, so it is as independent as we can make it. You can also use your up/down arrow keys to scroll through your keywords to see the details. Right now you can save 20 keywords and have it check all 20 with a single click. Pick your top keywords and track the up/down movement from day to day using the same 'scale'.