different cross section for same process generation

I generate an event, i.e. generate mu+ mu- > h t b~ mu- vm~, h > b b~ , ( t > b w+ , ( w+ > j j ) ), without decaying through madspin, and then I generate the same event (mu+ mu- > h t b~ mu- vm~) and have it decay through madspin, but I get a different value for cross section when I decay with madspin. Why is this? and how can I compare/convert between the two cross sections?