More data points from family and colleagues:
100% bonus - purchased miles twice within the past year
50% bonus - purchased miles once within the past year
50% bonus - never purchased miles
25% bonus - purchase miles once within the past year
Don't think the miles are based on status, miles purchased or miles flown as the owners of all the listed accounts fly similar routes, and do not have status with US as it is not used as the mile earning program for any of us