If you use just the TAB key without setting tabs, or, worse, use the space bar to align text, then the text will not line up correctly if you add additional text, change the font or text size, or copy and paste the text into another document.
As a general rule, you should never press the TAB key more than once between any two blocks of text. You should always set a tab position instead. You can always tell if a list has been created properly by turning on Show/Hide and revealing the formatting marks in the document:

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