744F and 744 fuel burn differences for GE engines

[phil@LFPG]

hello guys,

there is a long time before i posted here but here s a good question (on my opinion and i share it...lol)... while i was trying to create a profile for FOC 2003 for 744F and comparing with a 744 already done i was wondering why the 744 and 744F are not burning the same fuel in same time from the books ?

i mean of course same engines for both CF6-80C2B1F same zfw and of course same level and weight

744F
enroute climb tow 390T 19' FL310 120NM and 7900 kgs burnt

744
enroute climb tow 390T 19' FL310 121NM and 8000 kgs burnt

now the cruise

744F
.85 FL 310 380T TAS 499 12716 kgs each hour
LRC .852 FL310 380T TAS 500 12776 kgs each hour

744
.85 FL 310 380T TAS 499 13271 kgs each hour
.86 FL 310 380T TAS 505 13164 kgs each hour
.87 FL 310 380T TAS 510 14010 kgs each hour
LRC .859 FL 310 380T TAS 504 13128 kgs each hour
CI100 .845 310 380T TAS 495 13129 kgs each hour

so we can see the -400 is more burning than the -400F but getting better speed.
in the same time at .85 there is 555 kgs difference.
is the differences due to the upper deck or else ?
why the same burn or close too between LRC and CI100 for the -400 ?

hope this is clear ??

any pilots or engineers to help

truly yours

phil