Short version:
1. Revisions can really make things unpredictable.
2. How closely economists agree on the expected number can cause variation.
3. Other report released earlier in the day or at the same time can contribute to market movement.
4. Sometimes the market will do totally unexpected things. The market is driven by the buy and sell orders of huge numbers of traders. The signals are just Sir Pipsalot's best estimate of what the reaction should be. He's right more often than not, but no one can be 100% accurate in this sort of prediction.
Long version:
How to trade the FPA's Forex Daily Trading Signals