There are several factors at work. First, the range of minutes depends on more than just one week. Secondly, the GS doesn't automatically go up or down. Depending on how many minutes a guy plays and how many he has played in the preceding x number of differently weighted weeks, the final GS could be any of several numbers.
As for training GS, where you really see its positive effect is when you train it for two or three consecutive weeks.
Once I scored a basket that still makes me laugh.