It's perfectly normal for "semi-insular" country and a popular destination in Q2 + Q3

Analyzing a bit the A3 schedule you will see that there are many domestic routes that are beating the international routes in term of Available Seats - no matter winter or summer. In Q2+Q3, the domestic AS is even higher than in the rest of the quarters.

Some roughly calculations bellow.

S2019, ATH JTR ATH
6 * 320 / 170 seats = 1020
4 * DH4 / 78 seats = 312
1 * 321 / 196 seats = 196
1528 * 2 = 3056 seats per day

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S2019, ATH SKG ATH
9 * 320 / 170 seats = 1530
1 * 321 / 196 seats = 196
Total 1726 * 2 = 3452 seats per day

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S2019, ATH HER ATH
7 * 320 / 170 seats = 1190
1 * 321 / 196 seats = 196
Total 1386 * 2 = 2772 seats per day

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S2019, ATH JMK ATH
5 * 320 / 170 seats = 850
3 * DH4 / 78 seats = 234
Total 1386 * 2 = 2772 seats per day

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S2019, ATH RHO ATH
6 * 320 / 170 seats = 1020
Total 1020 * 2 = 2040 seats per day

Total for this 5 routes per day = 14.452 seats available

In Q3 there will be 92 days. So, 1.329.584 seats with only 5 routes.

In Q3 2018 there were 5.732.000 total seats. So the routes above represents 23.20% from the total of Q3.

I made the calculations using the max number of seats per plane and not deducting the eventual business class seats on certain flights.