3 Mobile Phone Tracking

[Mobius1]

Where I work we have the ability to track most mobile phone origin points to within a given area. I assume that this process uses signal triangulation in order to show a general area. Sometimes it will show a coverage area of an entire city or town, sometimes it will show an area confined to a few "street blocks".

Over time I have noticed that when callers using the 3 network call, the margin of error and the approximate area tend to be much larger and therefore less accurate than those of other networks. Is this because they have fewer mobile phone masts than the other networks or is there another likely reason for this?