mr minhas has right concept...

there is term called relative spd... when one jumps frm earth he's jumping in relation to spd of earth n his own... these two spds resultantly bring him back to same point of origin.

in case of someone jumping from top of roof of car, phenomenon is little changd where one jumps in relation to spd of car, earth n his own... tht results for a guy to fall backward of the car...

now let me clear it otherway discarding earth's movement... wt happens to someone jumping inside of moving train or plane.. he returns to a point where he jumped frm.. wt hppns to fly flying inside of a moving train, it kps flying wth its spd n nvr collides wth the walls of train... now similarly if one jumps inside of car, instead of jumping from top of car/train/plane, he will fall exactly on same point where he startd frm.. in all these cases one jumps or moves wth direct relation to the movement of othr moving body.. bt when one jumps directly exposed to car/plane/train n earth... he will fall on a point tht will be resultant of all movements of all three bodies.. so in case of car above in question, the resultant is backward involving car's spd, man's spd, earth's spd n effect of wind to some extent..

hope it gvs some idea.. ciao