כִּֽי־עֲוֺנִ֥י אַגִּ֑יד אֶ֝דְאַ֗ג מֵֽחַטָּאתִֽי׃
Bo winę mą wyjawiam, a pokutuję za grzech mój.
Rabbeinu Bahya
The answer to these questions is that once one knows that one has committed a sin the very knowledge weighs upon one and produces the guilt feelings desired by G’d. When a person is in doubt about having committed a sin altogether his conscience does not bother him so much as he tries to convince himself that he may not have committed a sin altogether. In order to bring home to the person who is not certain that he has indeed committed a sin, the Torah, by treating this “maybe sin” as if it were more serious than the definite sin, hopes to achieve the psychological effect it desires. Having achieved this result the “maybe sinner” may resort to confession of his laxity in not making certain that what he ate or whom he slept with was permissible beyond the shadow of a doubt. This is why the person described in verse 17 is called אשם as if he were someone guilty of a serious sin. This is also why the word אשם, אשום אשם appears three times in verse 19 of our chapter. The matter is comparable to what our sages explained in connection with the sin-offering on the new Moon in Numbers 28,15 where the male goat is described as לחטאת לה' as “a sin-offering for the Lord.” Seeing the Torah had not mentioned any specific sin this offering was to atone for, the sages (Shevuot 9) concluded that the meaning of the words are that only the Lord is aware of what possible sin this animal is to expiate for.