PaulS:
Turns out, that for their recommended line width of 3/4" 19mm, then the two middle sensors are on the line.
Which makes detecting the edge of the line faster, since an edge is closer to one of the sensors. The sooner you detect a change in the position of the follower vs. the line, the sooner you can make a correction, and the smaller the correction needs to be.
Yep... they also use a cunning method to calculate a weighted (weighted by sensor position) value of sensor readings, so you know where the line is relative to the sensor array. Upfront, you hand calculate the target value for the line in the centre, then hardcode that value onto the sketch. Then in real time you compare the actual value to the target, and steer accordingly. So in fact, the odd vs even question is irrelevant.