The main goal of IUPAC names is to keep them simple and understandable by anyone. So, you basically want to keep numbers the lowest possible. Hence, you shall start numbering where it will give the lowest numbers for your substituents.
For instance, if you began numbering from the carbon bonded with the chlorine, your molecule would have been named
1-chloro-6-methyl-3-nitrobenzene. Or, if you chose your numbering to begin from the other group, you'd have named it
3-chloro-4-methyl-1-nitrobenzene or, even worse,
5-chloro-4-methyl-1-nitrobenzene. With these examples, you can notice that the simplest name, i.e. the one that has the lowest numbers, is the one PubChem gave you : and that's the reason one should start numbering by the methyl.