Changing by a little is completely normal, but changing by a lot might indicate something is going on. This is the basis of inferential statistics and the logic behind hypothesis testing
Sample means always wobble a big by chance and the standard error measures how much.
In a pie chart, each category is represented by a slice of the pie. The area of the slice is proportional to the percentage of responses in the category.
This whole chapter helps makes the data I track for clients a lot more comprehensive. I use pie charts to help visually show which human in a family is working with their dog.