Unlike modern anime that often splits cours into separate seasons, Season 1 of Attack on Titan (aired April to September 2013) tells one continuous, brutal story without a mid-season hiatus.
Let’s decode the final part of your query: "13".
This is likely not an episode number, as the range states 1-25. Here are the three most probable interpretations:
Original Season 1 had production inconsistencies—some CGI Colossal Titan shots look soft even in 4K. 720p naturally masks those era-specific limitations while keeping hand-drawn Titan fights crisp. The 13 GB total means you can store the entire first season alongside two movies on a 32 GB flash drive.