The 75th percentile is a more robust intensity value to normalize the data. With any tissue that you are analyzing, there are a certain percentage of genes that are not expressed. Even if a gene is not expressed, a probe matching to that gene will still report an intensity value. These intensity values will most likely fall into the lower percentile ranking on the array. These values are considered noise and are less reliable. So, taking an intensity value of a higher percentile like 75th is taking a median of only the probes with reliable intensity values (taking median of only genes that are expressed).