It is funny when you said you are not complaining, just curious because my first thought was WOOHOO! Non foul smelling under arms is a great thing. Okay, now that the joke has surfaced lets try to examine the reasoning. This is a great question that needed some research.
The Eccrine Glands are the cause of perspiration under the arms. The Apocrine Glands secrete sweat through the hair follicles which causes bacteria and proteins – turning into acids - which ultimately cause the odor that we secrete. Sometimes it depends on what you eat which causes different foul smells such as garlic (yummy), that causes different odors. Did you know that even stress will cause different smells? Hum.
Anyhow, as lucky as you are, you may be lacking in the excessive bacteria that causes the odor in the first place although you are still sweating. My ex-husband never used deodorant and never was foul smelling. He was in the Army and worked out daily which caused sweating but luckily, he never had underarm stench.
As you said, there is no complaints but it appears that this may be the reasoning.