Looks like you were right about the game taking cup matches into account. The final of the state cup is played on the last weekend of the season, and I was counting on the final round of the league to be played on that very last weekend. This is where the game seems to have realized that it is not possible to play a cup match and a league match on the same day, in case some of the teams participate in both competitions.
i solved it by setting the start of the second half of the league to start a week earlier, and that way i got what i wanted, 15 league rounds in each half season.
Thank you very much for your reply.