Or HaChaim on Numbers 5:28/אור החיים על במדבר ה׳:כ״ח

1 לא נטמאה האשה וטהורה היא ואם, And if the woman had not been defiled and remained pure, etc. The reason the Torah repeats that the woman had remained pure is that the Torah wishes to tell us that not only has she become exonerated so that the waters will not harm her, but she shall even conceive seed. The additional word וטהורה also indicates that not only had the woman in question not been guilty of intercourse with someone other than her husband, but she had not indulged in any of the caressing and fondling which usually precedes sexual intimacy. When both these conditions exist she will be blessed by becoming pregnant in accordance with what the Torah writes. If, on the other hand, לא נטמאה, she had not become defiled through actual intercourse but could not claim to be "pure" by not having indulged in foreplay, she will not be blessed with child although she will escape punishment for her conduct.

2You will note that the priest who made her take the oath (verse 19-20) employed somewhat peculiar language. What did the Torah mean with the words כי שטית? If it means becoming defiled why does the Torah write וכי נטמאת, "and you have become defiled with the conjunctive letter ו? Clearly the word refers to something which precedes the actual defilement of a woman through intimacy. The priest cautioned: "if you have actually become defiled, then you qualify for the punishment provided by the Torah." When speaking about the reverse situation, i.e. אם לא שכב איש אתך, "if no man other than your husband has slept with you, etc.," then there are two possibilities. 1) You are exempt from the damaging effect of these bitter waters; 2) you may be totally exonerated and benefit by the blessing of becoming pregnant as promised by the Torah. The third possibility ("she is pure" as distinct from "she has not been defiled") can be understood as our sages say in Sotah 18 וטהורה היא מאיש אחר, that she is pure "as far as having slept with another man" (not the one her husband suspected her of) is concerned. According to our approach, "the other man" is already included in the statement that she has not been defiled which includes the man with whom her husband suspected her as well as any other man who is not her husband.