Color of fried beef steak: sometimes dark, sometimes light-colored. Why?
It's quite likely that the steaks that ended up darker were dry when you started cooking them.

If you don't dry off the surface of your steak, the heat is used to evaporate the moisture on the surface, which ends up steaming the steak rather than developing a good char/crust.

The darker colors are the result of a chemical reaction that results in more flavorful compounds being created.

As for the juiciness, the higher heat (as there wasn't evaporative cooling) will brown the outside faster while leaving the inside more rare (and therefore moist/juicy).  When it steams, we leave it cooking longer, which can result in the middle overcooking (squeezing out moisture).