How come I only have a six pack when I flex?

Ok so I'm a skinny guy and really the only time you can see my six pack is when I flex my stomach muscles. I did a body fat percentage calculation and iirc it's like 11%. What can I do to improve this? I eat subway a lot and I've started eating a lot of salads instead of cheeseburgers. I can pretty much eat anything and not gain a pound but I was curious as to what I should be eating to make my six pack show without flexing?
How come I only have a six pack when I flex?
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