dbl. monitors in systray. What do the mean?

I have asked this at various places but never got a satisfactory answer. I know they indicate an internet connection but the point is at what speed?
Some say they indicate the initial connection speed established with your ISP but that this is only a 'rough' guide and not always reliable. At speed test sites I've tried the results are always diff. to what the "hover over the monitors" box indicatetes.
Others say they are indication of internal system speed, ie, the speed at which is incoming data is processed in the machine but then why does it say X Kbits data in X Kbits data out at this or that speed. Anyone know? I belive zillions of people use the hover the pointer over the monitors to get their internet speed but we are all wrong!

Any ideas. Any Gurus, anyone even hazard a guess would be nice. Cheers. m