The dry hot summers are caused by a change in oceanic pressure systems. Early Greeks lived on the outskirts of the land, always wanting to be as close to water as possible in order to make transportation and fishing easier. Because of this they may have had a more constant climate that they would have inland. This is due to the cooling properties of water and land. So around the water you receive a very softened climate.
Water heats up slower and cools slower.
A solid object on the other hand heats quicker and cools quicker. Mountain regions in Greece as in any place have different climates from the places which surround them. As altitude increases it is like going up in latitude. It generally tends to get cooler. This cool air causes the air forced up over the mountain to cool also, causing it to lose its ability to hold its water.