It says about you, what you say about it. Handwriting is a reflection of your behaviour and personality.
If your handwriting is really inconsistent, then it means that your behaviour is inconsistent and the opinion about you in the minds of people is also inconsistent.
This makes you talented but unpredictable and many times unreliable.
If your handwriting slant is inconsistent then you are a moody person. Sometimes you would feel like talking to everyone and other times you are withdrawn and prefer being alone.
If the writing size is inconsistent then your confidence is wavering. Sometimes you are at the top of the world and sometimes you are in self doubt.
If your handwriting pressure is inconsistent then your energy levels are inconsistent. Sometime you would feel like doing lot of work with complete engagement and sometimes you would just work casually without much involvement and energy.
So it all depends on what kind of inconsistency is there in the handwriting.
If you are talking about slight variation in handwriting from time to time then it is a sign of being a human being. We have different moods which affect our behaviour. If handwriting reflects our behaviour then these variations will also be seen and should be considered normal.