Although mock drafts can be a guide for your actual fantasy football drafts, remember that a variety of factors will affect how and when players are selected in each round.
Timing is a big part of it. If you drafted at the beginning of August, there was likely more optimism surrounding Andrew Luck's leg injury. Halfway through the preseason, Luck's Week 1 status is now more in question, which has a trickle-down effect on his average draft position and his teammates' ADP, too.
You also must take your league's scoring system into account.