The cocking ramp dictates the range of motion of the firing pin moves within the assembly. The top-forward position of the ramp defines how much the firing pin is being retracted, and the bottom-forward portion of the ramp defines the resting position after the trigger is pulled.
The cocked position is dictated by the location of the sears' top tab in relation to the cocking piece when the bolt handle is lowered. Of course, the position of the top sear tab is in relation to the bottom sear tab which engages the trigger mechanism. A slight change in the angle to the bottom sear tab will have an effect on precisely where the top tab will catch the cocking piece, thus affecting the position in which the firing pin is retained. As such, a bad sear can result in either an over-cocked or an under-cocked condition.
So say the cocking ramp in the bolt body is allowing the firing pin to be retracted to much (one form of over-cocking). That alone adds additional resistance to opening the bolt as you're putting more tension on the firing pin spring. This is why some people are under the false impression that a lighter firing pin spring helps the timing. With the lighter spring there's less tension and thus easier bolt lift. The only problem with this is that you're also greatly reducing the energy enacted upon the firing pin to strike the primer as a compressed light-weight spring will have less energy than a heavier one when compressed the same amount.
Now when we close the bolt some of that cocking distance and tension is released as the bolt is rotated and the cocking piece drops down and is held rearward by the top sear tab. This is where the sear comes into play. If the bottom tab that engages the trigger is off it will affect that engagement point on the cocking piece. (don't ask me what the corresponding degree to distance ratio is between the tabs as I have no clue) So if the bottom tab is off X degrees, that will move the top tab forward or back Y amount, thus affecting our cocked position (easily resulting in an over-cocked or under-cocked condition) and also the firing pin travel (less or more tension on the spring = less or more energy to move the firing pin).
Getting back to the hard bolt lift issue, it's all about the timing of the interaction of all these components. The majority of bolt-opening rotation is used to retract the cocking button up the cocking ramp which in turn allows the sear to rise back up toward the cocked position. The last part of the rotation applies the primary extraction which pulls the bolt back slightly allowing the sear to fully reset and engage the trigger mechanism. These events have to all happen in proper sequence, and if one or more event happens out of sequence it will result in the drag which causes the hard bolt lift.
Given the varying tolerances of all the parts and the fact that these are mass production rifles it's easy to see how and why they have timing issues.