I mentioned theories about it few times - way less visible archer paradox and bolt flexing in general, causes less energy waste, both on bending and due to air resistance.
Thus, bolt preserves it's velocity better, especially at 'flat' trajectories.
Dunno how true it actually is for 'most' applications, would need checking.
Theoretically, velocities possible with crossbow, might be, all other things being roughly equal, greater than with 'long' bow, because both arms have way less road to cover.