A few things you should know about hardwoods:
Top veneer hardwood (5)
bottom veneer hardwood - typically these should be made of the same hardwood
w/polyurethane, titanium oxide, aluminum oxide or oil finish(2)-(4)
Cross layered hardwood - 3 or more layers depending on the manufacturer
By changing the direction of the plys, expansion and contraction due to temperature and climate changes are equal on all four sides giving engineered flooring more stability than traditional solid hardwood floors.
Most woods today are engineered, like the picture above. This is done to help reduce waste and cost, but also to help with stability of expansion and contraction of the wood.