help : activities total float is increasing by decreasing the remaining duration for the activates

hello all ,

i'm working on a big schedule it's about 20,000 activites  and by updating it
i usually notice that  for some linked activtes package  the activity total float is increasing by decreasing the remaining duration for the activates .

i will explain  it more as there is progress on site i update the progress percentage complete for the activity and also decrease the remaining duration ( physical type) and then the finish date for the linked activites shifted to be more than the current date instead to be less 

noting that i don't use leds or level of efforts activites and neither SF relationship  (only SS and FF  or FS relationships)

and the activites don't have any ONGOING OR NOT STARTED predessesor only the relationships between them selves 

thanks in advance