Can't get right branching ratio

As a 1st step I checked the W branching ratios, comparing (w+ > all all) and (w+ > j j), (w+ > l+ vl), (w+ > ta+ vt).
It gave the right ratios (the same as the PDG).
Then I wanted to check for the (p p > t t~) process :
In my first proc_card I set p p > t t~ / (t > w+ b) / (t~ > W- b~), I obtained a xsec of 483 pb (for beams of 6.5TeV).
From this, for a leptonic decay of both Ws I should get xsec = 483 * 0.045 = 22 pb, but changing my proc_card into
p p > t t~ / (t > w+ b, w+ > l+ vl) / (t~ > W- b~, w- > l- vl~) and generating events I obtained xsec = 17.6 pb.

I can't figure out why I don't have the good result, I let the basis parameters in the run_card so it shouldn't come from different parameters between the runs...