Handwriting is a subconscious activity. If I ask you to write an essay on the topic “The most memorable day in my life.” you will start thinking about the subject and the content of the essay. While writing the essay your focus will be more on the words you are using and sentences that you are framing. Occasionally, you will also think about the correct spelling of words etc. Do you think about the handwriting formations, the slant, size, pressure, word spacing, letter spacing, etc? Do you think about how the t is getting crossed or i is getting its dot? Not so much. It is not possible to be aware of both the content and formations of the handwriting both at the same time.
Your handwriting formations depend on subconscious patterns that you have. Handwriting is affected by your moods, health, emotional state while writing.
As humans we have some variation in our behaviour so that is reflected in our handwriting too. You won't find your handwriting to be exactly the same everytime. If your handwriting is the same for years and it does not change a bit at all then you are probably a machine or live a life like a robot.
But if your handwriting varies too much in a short span of time it shows that you are too moody, unpredictable and unreliable.