Aanaa,
I believe the Universe is a place of infinite possibilities. If you study the quantum theory of physics, you will get to know that in any given situation, an infinite number of possible future situations exist, each of which has a certain probability of happening depending upon the previous events (
http://en.wikipedia.org/wiki/Schrodingers_cat). The same thing is true about our future.
" Ellaame nadandhu mudindhadu " means "All possibilities exist and have happened/can happen when a certain sequence of previous events occur." That sequence (of previous events) is primarily determined by the actions of the individual. Just as in quantum physics there are an infinite number of variables to be taken into account whenever one has to study the behavior of any particular particle in a many-body system (
http://en.wikipedia.org/wiki/Many-body_theory) , in the same way, the future of an individual involves so many variables, the most important of which is his/her own efforts. Quantum physics therefore is a good analogy, however I do not think one can proclaim for astrology to be a science in the normal convention of the word, as Quantum physics is. I think that would be a bit preposterous.
Predicting the future means picking one of those infinite possibilities of the future. Therefore it is impossible to predict the actual sequence of events exactly (picking one with 100% guarantee would mean that the Universe is in fact not a field of infinite possibilities and there is therefore no meaning for human effort). Thus the astrologer, however good he/she might be, can only indicate those events which have a high probability of happening. Again there is a small albeit finite probability that it might also not happen, because of the fact that one cannot track all the infinite variables involved (again, the most important of which is human effort). I believe this is what is meant by "vidiyai madiyaal vendru vidalaam".
Love and Light.