3-ethyl-2,5-dimethyl-4-propylheptane 

Here, we can see that there were several possible 7-carbon chains.  The one used here as the parent (blue numbers) is the one with the highest number of branches.  See how many other 7-carbon chains you can find. Note that they all have less branches on them and that means there will be sub-branches on branches.  Hence, the molecule shown uses the simplest, i.e., correct way of numbering / naming the compound.