Why Tarot readings are not always accurate? ... 
Is there a surety of a 100% in this world? Pundits have a catch phrase that, "You do more for your friends than that compared to what they do for you!" OR "You earn a lot of money but it does not stay with you!"
** Ha Ha Ha!! **
There are as many reasons as one can think of. Some of them could be mentioned as, distraction of any kind during the reading, the concentration of the enquirer and that of the diviner should have been optimum. The seriousness or faith of the enquirer in regards to the question/ issue or in regards to TAROT reading it self.
In most cases the diviner, has a very importantly role to play. Most readings are done from a point of view of the diviner's understanding of the question / query. There are conditions that may not be always favorable for the diviner.
Moreover, since the Tarot readings provide an insight about the future, I have often observed that people get warned in advance or they are careful. This creates or initiates an entirely new chain of events which ultimately determines the outcome. For example I had told an enquirer that while walking down the road to work, you will fall and get hurt. That day she hired a cab and reached office. She was not walking so she did not fall and get hurt.
There are cases of bias from the diviner or the enquirer or in fact both of them. The bias will be discussed in "Advanced notes on TAROT reading"1.
1. Advanced notes on Tarot will be pulished in this web site soon.
Disclaimer: The TAROT card readings provided by me are for a research purpose only. What I can provide is advice only and the readings are indications of what might happen and are not craved in granite or inevitable. You are the captain of your own ship, it is you who need to decide which course to take. I will be NOT responsible for any loss or negative impact on your life/ property/ the area of concern/ problem or complications as a result of the reading/ inputs I provide!