Reddit reviews The Balkans: A Short History (Modern Library Chronicles)

Lots of people who live in northern Greece today are descended from Slavic-speakers who lived in the same place in the 1800s. Also, many Slavic-speakers were forced out in the early 1900s (or left of their own free will in many cases).

Also, many villages in Macedonia were ethnically "converted" to Greek, Bulgarian, or Serb identities during the Balkan Wars of the 1910s. This meant that the incoming militia would round up the local population and force them to write little oaths that said that they were henceforth Greeks (or whathaveyou). Source: Mark Mazower, The Balkans: A Short History.

If you want to learn more about the establishment of Greek national identity and the mixing of "Greeks" "Turks" and "Slavs" in Macedonia, I recommend this book.

You're right that many Greeks have nothing to do with Slavs (and especially not today where both Slav and Greek nationalisms have pulled them further apart) but in the 1800s the lines were far more blurred.

The Balkans: A Short History by Mark Mazower is a good start.