Possibility A:
If you had braces back in the days, then your 4 of your premolars (two on top and two on the bottom) have been extracted for your braces adjustments. This is counting that you have your 3rd molars intact to make 28 teeth.
Possibility B:
Your 3rd molars (wisdom teeth, 4 of them) may not have:
1) erupted yet
or
2) they may be impacted
or
3) they have been resorbed by your Odontoclasts (tooth-eater cells) and/or Osteoclasts (bone-eater cells) within your jaw bone prior to eruption [not visible under x-ray films]
These two possibilities are my educated guesses, then again, I'm only a dental student and not a dentist yet, I may not know what I am talking about!
😀