My 15 mm invasive ducal carcinoma was not detected by any of the 11 mammograms I had - the last one only a few months before I felt the lump itself.
This is why it is so VERY important that people do self examinations and not rely on screenings. Sadly no test is ever 100% accurate - there will always be some false positives and false negatives due both to errors and limitations of the test procedure itself.
Don’t dwell on the what ifs - be grateful that your cancer was detected and encourage others to do their self exams.
Good luck with your treatment.