75% are guaranteed to be intact down clusters. The remaining 25% can be down fibers or feather portions. A product can be labeled as "minimum 75% down" and still meet the 750fp advertised, but manufacturers might want to label it with the 85% minimum or 90% minimum needed to reach 750fp.
The minimum 75% down label can also be used on 450fp and 550fp. US law does not require Fill Power claims to appear on the law label. For instance, duck down meeting the law label requirements for 75% down might be 500fill power, or to stay within the advertising limits for fill power (a few % tolerance) might be advertised as 450fp.