GPS status screen



On the GPS status screen, some satellites are shown in blue and some in grey. The right side of the screen shows all satellites plotted on a scale of 0-100. I assume this shows the relative strength of received GPS signal.

I had thought that the blue satellites had a stronger signal than the grey satellites (and therefore were used for the position calculations), but often a grey satellite shows a higher value than a blue satellite. What do the colors mean?
Per the manual:
Satellites used to find position are marked with a blue octagon.
Satellites not used to find position are marked with a gray circle.