Why are there Meccan verses in Medinan surahs or vice versa?

The Meccan Quran is what was revealed before the Prophet's emigration (peace and blessings be upon him), even if it was not in Mecca, and the Medinan is what was revealed after the emigration, even if it was in Mecca. Many surahs were not revealed all at once. A single surah may be both Meccan and Medinan, and a surah is labeled as Meccan or Medinan based on its beginning. Ibn Abbas said: "When the opening of a surah was revealed in Mecca, it was written as Meccan, then Allah would add to it whatever He willed." Both Meccan and Medinan revelations have objective criteria and characteristics that distinguish them, and this can be known by referring to books on Quranic sciences.