Their platform has core values, and everything flows from that. The end results do tend to match the opposite sides on the surface.
For example, drug legalization. Libertarians want it because of the concepts of limited government size and power, freedom of the individual, and individual responsibility ("I was high" becomes an aggravating factor in a crime, not a way to evade responsibility). The concept exists even if a person, like me, is against the use of most recreational drugs (you're not taking my caffeine!).
The leftists just want to get high and be excused from responsibility for anything stupid they may do when high.
It's a big policy difference when you look closer, but most people don't.