Welcome @eva.chen! 
Congrats on your self-initiation and full year of magick with Spells8- wishing you many more magickal years to come! 
That’s a good question! Like Benjamin said, environmental factors could have had a role in causing one candle to burn faster than the other. If you don’t think there was an outside force causing it, then I think @Phoenix_dawn’s suggestion of reading the wax could be a good next step in your search for meaning.
The Candle Magick 101 guide says the following about candles 
:
Candles symbolize light and the Spirit being brought to life through the Element Fire, burning in the hearth , a symbol of home.
As this candle was part of your initiation, I’d recommend looking for signs related to your path into magick, a potential spirit guardian or spirit angel reaching out to you, or contact from a deity.
As you study your candle, feel free to take a look at the following threads with tips on how to read candle wax:
→ My Wax Reading
→ My First Wax Reading
→ Tips on Reading Wax in the Flickering Magic Guide
Congrats again and blessed be, Eva!