also genetics, some people just have a higher metabolism for no reason, means calories are burnt quicker even when resting, less weight gain.
also about being visibly fat, rather than muscly, again, altho working out helps, some of it is down to genetics and how ur body was treated in its early years to how it uses muscles/fat.
some people naturally have super low or high body fat percentage , those people with naturally high body fat percentage, although they will be able to lose weight and lower there fat percentage. theyll never have the washboard abs(well they will, but it will be invisible under an inch of fat).
this is down to either genetics, or the bodies upbringing, and its requirements for energy as a youngster.
for example, if you were a person who was mal-nourished as a child, and were very thin. chances are the way your body works now, is to absorb all the energy it can from food, which it will burn slowly over 2 or 3 days,(rather than turn it to fat) as it knows turning it to fat is pointless as it will be needed soon, as youll often miss days without meals.
as to wether its genetics or up bringing, cant be proved, as any tests like this on genetically identical people (mono-zygotic twins for those in the know) would be considered dangerous and unethical.
yeah workin out helps, but some people, no matter how hard they try, will never get washboard abs, as there body will use protein and even muscle fibre for energy, before it will drop the fat levels down to 8-9% body fat(which is the level where washboard abs would become visible).and by the time you reach this,the muscles will have wasted(atrophy) to the point wer no abs are visible.