they're possibly the least accurate thing in existence.
GCSE's my targets were B's-D's and I got A*-B's in essentially everything.
In AS my targets were A for maths, and B for the three sciences. I ended up getting a B in maths, B in chemistry, C in bio and D in physics.
Also AS is COMPLETLY different to GCSE.
In answer to ur target being low is probably to not dishearten you by you're very likely poor marks at the beginning of the year. Not insulting you, but my targets remained high and going from A's in all tests right down to E's and U's in everything for ALL four subjects really got me panicking. I was thinking "how the flippin hell am i going to repair this, i'm flippin doomed" which caused my grades to remain low. it wasnt until later through the year when everything started to click into place that i got out of my misery and actually started to rise again. i definitely feel i couldve done a lot better if i had a lower target.
Long story short, ignore every target grade you have when you begin AS, it's about January time when you should think about it.