Bill Mitchell replied to this question in another thread and I am paraphrasing his answer for you. There are a lot of reasons to why the 75% Color Window Patterns are better.
1. 75% Stimulus Color is closer to the more typical use of the display thatn 100% Stimulus Color, which is more the exception. It's more important to get the 75% color right.
2. At 100% Stimulus some sets start to deviate from the behavior at lesser stimulus, the RGB gamma becomes more unbalanced, color temperatures of white change, plasma voltage regulating circuitry starts to kick in, etc. At 75% Stimulus, it is more likely that you are working with a purer white and more representative color gamut. You are more likely to set everything more correctly for a majority of the display behavior using these patterns. Your 100% points may end up being off from the reference, but the rest of the time the set would behave correctly.