Terrestrial Day vs. Venusian Day: Why the Huge Time Difference?
There is no reason why the day length should be the same. From the time of planetary formation, some 4.6 Bn years ago there has been some degree of slowing of day length due to tidal friction, which is much higher on Earth than on Venus, so the original contrast in day length may have been even greater than today. 

There are three major reasons for differencial spin rates. Firstly, the initial planetary rotation rate was a function of the net angular momentum of the planetesimals, gas, and miscellaneous debris from that part of the solar nebula from which the planet condensed (in accordance with the conservation of angular momentum). There is no reason to suppose that that the original solar nebula consisted of a nicely ordered, evenly distributed mass/velocity distribution. Rather, there is every reason to think that it was a highly chaotic heterogeneous cloud of matter undergoing gravitational mayhem - probably including large scale gravitational disruption by gas giants much closer to the sun than we see them today. From such chaos one could model almost any distribution of relative rotation speeds amongst the final planets.

Secondly, there is strong evidence that proto-Earth collided with a 'trojan' named 'Theia', about the size of Mars, about 4.3 Bn years ago. This is the 'giant impact hypothesis', which provides a good explanation for Earth having such a large moon. We don't know the relative angular momentum of either proto-planet, their relative motions, or the angle of impact (head on? glancing?), so it is probably impossible to reconstruct the event. However, it is (relatively) easy to envisage how such an impact could radically change the Earth's angular momentum. The spin could have been hugely accelerated or decelerated according to the physics of the impact. 

Thirdly, Venus's relative proximity to the sun could have resulted in its undergoing rotational capture, such that eventually one face of Venus will perpetually face the sun. This gravitational locking of Venus is probably the most significant reason for its' long day. As you rightly note, the retrograde spin of Venus is curious. Therefore we may infer that Venus's spin history is more complex than we can account for by simple planetary accretion.