I'll give it a shot... but I'm not sure my minute explanation will suffice.
Code:
B1-------B2
| |X/
| |/
A1---A2
Two ships, A and B, shown at their locations for Time 1 and Time 2. Ship B is faster, and they're sailing parallel. At time 1, ship B is directly north of ship A (angle Y - not shown). At time 2, ship B is north-northeast of ship A, which creates angle X. We compare the two the two angles, and for every 1 degree (on a 360 degree circle) of separation per second you lose 6% accuracy. Actually, that value is now 4%.