Sometimes children are taught a particular way of laying out a question that works, but they have no idea why.
Others are taught to do it more slowly/in a more complicated way, so that they understand it, and then they move on to an easier layout.
Children who can do the second type of layout aren't necessarily understanding why - they might be, or they might not be. If they aren't, it leads to problems later.
One example - children are taught to 'balance' algebraic equations by adding 3 to both sides, or whatever, which means that a 'minue 3' on one side is cancelled out. Eventually, they realise that this is the same as move that term to the other side of the equation and changing the sign, but they understand why this is so. If they are just taught to move it to the other side and change the sign as a rule, then they get confused later on about why and how, and often forget to change the sign.
Or cross multiplying fractions. This works if you have fractions on both sides of an equal sign, and no other terms. You can just cross multiply, and it will work. But children who don't know why this works tend to overuse the technique and start cross multiplying any two fractions they see near each other! They don't understand that they are just multiplying both sides of the equation by each denominator and cancelling. Eventually, once they understand that, cross multiplying is a qucik way of doing it.
Going back to basics can also prepare children for a harder topic. Usually they stop doing 'grid multiplication' in the middle of primary school, and use a more efficient column method. But in secondary school, they might revisit the old grid method to multiply two-digit numbers, so that they then understand why they have to mutliply out double brackets by multiply both terms of the first bracket by both terms of the second.
They might go back to using tables of values and plotting co-ordinates, in order to understand what the gradient and y-intercept are in the equations. Later on, they can plot the lines using just the gradient and intercept, but sometimes they might be asked to specifically do this using a table of values, as it shows other skills and promotes understanding of things like where the points on the line come from and how to tell if something is on the line.
Those are just a few examples of things I can think of where doing a more advanced method might not be what the teacher is wanting them to do at a particular point. It could be that they children are far enough ahead that they do in fact understand the concept well enough to be using the quicker or more efficient ways, but the teacher won't necessarily be able to check that, so it's easier to get them to do it the way everyone else is, to make sure gaps don't develop.