or 
If we could answer that easily then there would be no need for LYW (lyw4.life)! The reality is that we all learn differently. What works for one does not necessarily work for another. Take something as simple as subtraction. There are two methods. A simple sum, such as 52 - 27, a primary school level task, can leave graduates confused. Seven from two will not go so we need to put the 1 in front of it. So far everyone agrees. But then some people will get that 1 by adding another one at the bottom, making that 3, whilst others will have been taught to subract 1 from the top giving a 4 there.
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Both methods give 25 as the answer, but using one method will confuse those who learnt the other. This is not the fault of the person presenting the work. Obviously, it is not the fault of the person being taught. It is just a normal misunderstanding - one which leaves the learner confused.
If that can happen with something as simple as subtraction, imagine the scope for misunderstanding that occurs later. The more sources and the more diverse the strategies employed in these sources, the more like someone is to succeed.
How those who create input see the subject also has an effect on the style of the materials that they produce. The order of the materials and the context of the subject matter will benefit some and hinder others. Some like to jump into detail, whilst others prefer an overview first. Some need a familiar context, whilst others work better with abstract ideas.
Students all proceed at different paces. There will be those who find the early topics easy, but then struggle. Other people may struggle to get into a subject, but then find they breeze through later topics. And both these sets of learners might finish the course at the same time. However, conventional teaching methods demand a scheme of work which restrict all to broadly the same sequence and speed.
Next, the expectations of the teacher/lecturer will also affect learning. Expectations set too high will leave some feeling inadequate, whilst low expections would limit potential. Expectations set in the middle of these two points will hit the peak of the normal distribution (i.e. maximise the nuber who are enfranchised) but leave those outside the ability of that middle band with one of the outlying problem.
Finally, self-worth will also play a huge part. A large proportion of highly able students have been convinced that they are "not up to it" by a system that does not have time to discuss anything with them. There is a mis-match between the individuals actual ability and perceived ability and that unfulfilled energy has to go somewhere. They become a discipline problem, socially inept or get into alternative cultures that value them more such as gangs. Traditional outlets for this energy, such as sport and roaming the countryside pretending to be pirates is less easy now.