I am not sure if you mean that the standard varies from year to year or if you mean that the year of the program matters, but I have heard that the latter is true, although I don't have any way to confirm that the information I received was correct. If I correctly understand what I was told, what is deemed a high score depends on what year you're in. Using myself as an example, I received 18.5 and applied while in my first year of the PhD program and received funding for three years. However, maybe for someone in second or third year already, 18.5 might be an unacceptably low score. Again, I don't know if there's any truth to this, but it's something that seems generally accepted, at least in my department. Sorry I can't offer any more concrete information I have also heard that the threshold for an acceptable score is higher this b/c of cuts, but again, I don't think we can confirm this.
Sarah