At my school it seems that if you are getting A-B at AS you are predicted the same for A2, but if you get a C or less (sometimes even with a B) you get predicted lower - obviously depending on factors like re-sits, motivation and work ethic. I think it might be because if you're getting a B-C at AS, you're likely to find it harder to cope with the harder work at A2 compared to if you're already getting A's at AS then you're likely to put in enough work/be clever enough to cope with the A2 standard.
It also depends a lot on the subjects you're taking - english and history it's harder to get an A*, but you're more likely to improve a lot in essay subjects compared to sciences. You just kind of have to trust your teachers - they know your organisational skills and how hard you work (more reliably than our own perceptions of ourselves) and they've had the experience of students improving or going down.