I think the lower miles is correct but am not 100% certain:
"25% of the miles you’ve flown during the month."
"miles flown" would seem to be the key wording as opposed to miles accumulated or miles earned. Miles flown to me implies base miles but it would be great if they used the LH approach and multiplied ALL miles earned!
Does the mileage calculator work differently if you are an elite member and logged in? (ie does it show you your 25% bonus?)