Most people have various indentations in the skull. The skull is not one solid bone, but made up of several plates that aren't originally fused at birth, but do so later in development and up into adulthood. The one on the crown is usually caused by the "posterior fontanelle." At birth, this is the "soft spot," or area where the skull bone plates don't yet meet. It's covered with cartilage at birth, and will usually hardern as the child gets older. If the fontanelle is unusually wide or large, the filling in might be incomplete. It would be extremely rare for it not to harden at all. However, certain conditions are known to cause a sinking of the fontenelle in infants: dehydration, malnutrition, or it can simply be a normal variant. Most likely, if there was no injury, you've just never noticed this subtle, yet completely normal, imperfection in the past. If you don't believe this is the case, a CBC, blood studies, urinalysis, and imaging studies are needed to determine whether or not a nutritional disorder, or structural skull deformity is present.
No one's head is perfectly rounded, though some are prettier than others, to be sure. I had the same revelation in the same area a few years ago, and wondered how it was possible I never noticed it before, but when I review a profile picture of me as a baby (and without hair), the indentation was clearly visible. You can probably even see it in this picture of me here (lol):
The skull usually fuses completely by two years of age, and aside from blunt trauma or injury to the head (which you would have certainly noticed). There aren't any other conditions that would cause it to sink in unless you had a rare, incomplete fusion of sutures/fontanelle and were also severely malnourished or dehydrated. Growths on the brain or atrophy of the brain, or even having chunks of your brain removed entirely would not alter structure of the skull in this way... it's like a durable helmet. Unless you are having any other unusual symptoms, or have reason to believe you're severely dehydrated or malnourished, this is most likely a normal variant you simply haven't noticed yet.