No matter what browser, what OS, what connection, what whatever, the meter will have good days and bad days. For some, a very low percentage I guess, it is always good. For most people it is often decent. For some it sucks most of the time. If WGT had some guts they would make a survey to find out how these percentages spread out.
I have said it before and I say it again - they have simply put too much complexity in that meter to make it stable. Easiest proof of that is that it does not work in the iPhones either and there is no Flash there. Nothing to tweak either. There is no doubt that they are aware of the fact that their meter sucks. What reason they have to ignore it is a mystery.
WGT - get rid of those f**k**g gradients in the meter, it would in one go make the meter a LOT better for everyone!