The statements by Sepp Herberger were nothing more than stating the obvious: of course the ball is round, and everyone knows that a (normal) soccer game lasts 90 minutes.
There is no story or deeper sense behind that two sentences, just Sepp liking to give interviews and forming humorous statements.
People tried to put sense in these statements later that was not originally in there.
Both statements (der Ball ist rund and das Spiel dauert 90 Minuten) do not belong together and were made in different interviews on different dates.
They became a complete statement later as these were the most striking ones made by Sepp Herberger.
The meanings that were later added to the sayings are:
Der Ball ist rund
The ball can go in any direction, it is not predictable what happens in a soccer game
--> everything can happen
Das Spiel dauert 90 Minuten
In soccer one team can dominate a complete match but does not score even one goal.
Sometimes it happens, that the underdog then in the very last minute of a match gets one single chance and scores, and then wins the game.
Trainers then often use this statement, to say: it is not enough to play good 89 minutes, as every game lasts 90 minutes and the result can be changed within the last second.