Handwriting reveals the personality and behaviour of a person. Since humans tend to behave slightly differently based on their moods and circumstances handwriting will also vary. Actually, if there is not a bit of variation in handwriting with respect to time then the person is like a machine.
But your question is about a person having altogether different handwritings at times.
It tells that the person can behave differently at times. He is moody. He can be totally different from what some people know him as.
Since he is moody, he becomes unpredictable.
And since he is unpredictable he becomes unreliable.
He can behave very strangely different.
Some people have different writings for specific purposes. For example their writing in office documents is different from their writing in the grocery list.
These people carry different personality or public images in different groups.
Little bit of variation is natural. Altogether different writings is moodiness, unpredictability and unreliability. The person can be full of surprises for some.