(The smell which comes out of the mouth of a fasting person, is better in Allah's Sight than the smell of musk.)

[An-Nasa'i]

After many hours of Siyaam during the day, the smell of the mouth of the Sa'em (the one who observes Siyaam) becomes unpleasant because of refraining from food and drinks. This smell is better than musk in the Sight of Allah.
This metaphor in the Hadith is similar to comparing the smell of the finest fragrance wore by a sinner to the smell of the salty sweat of a hardworking pious person who strives in the Cause of Allah, so are they the same? 
When the Sa'em leaves food and drinks, the smell of his mouth becomes unpleasant, but this is the smell of obedience and closeness to Allah, and it is better than musk in the Sight of Allah