Now, I am no recruiting junkie, but I do like to look at the rankings, players, etc.
I see lots of questions asking various things about rankings, such as why they don't base them solely on quality / star ratings of players, why they don't focus more/less on number of recruits, etc.
They try to be as objective as possible.
If they were to take into consideration the size of your class based on the number of scholarships you have available, they would need to compare your class with what your needs are, which would quickly become very subjective.
Different services base their rankings on differing criteria, such as ESPN focusing on the number of players in their top player rankings.
More importantly than a single year's ranking is how you average over several years. Remember when USuC outranked us in the recruiting rankings and we kicked their a$$ on the field?