We know that the exact time is known by each cell-tower ....
Now, the discrepancy by where I work is 5 seconds, while the discrepancy at home is 10 seconds. Maybe it's a latency problem, possibly caused by some kind of round-robin polling activity that discovers and pings the phones within the tower's range. That's all I can think of at this point.
Some people have noted that their Windows phone is at least a few seconds, usually more, off when set to Auto update time.
The best hypothesis seems to be that your phone is setting itself to the time your Carrier sends out. This time is not important for electronic synching. This means local configs may vary and latency issues can be human noticeable.
This is a windows MP8 issue that is a near universal bugbear for Windows phone users. People have tested Windows vs Android in the same location on the same carrier and the Android version is basically spot on, suggesting they are using different sources for the time. This has been an issue with those phones for so long I wonder if doing it in this way is fundamental to the functionality of the OS in some daft way.