Why would someone get 7 Years for residue possession under 35 grams of meth ?

Since we do not know all of the specifics of the case, we would be making a wild guess as to why. Here are some thoughts. Past criminal history is a big factor, and so is going to trial and losing (prosecutors seek the highest possible sentence if they have to take the case to trial). If "conspiracy" was part of the charges, that might increase the sentencing guidelines.