This is because, apart from Awa-no-kuni, present-day Chiba Prefecture (the tip of the Boso Peninsula), Kazusa-no-kuni (the central Boso Peninsula), having the kanji meaning "upper," was in fact below Shimousa-no-kuni (the upper part of the peninsula), having a kanji meaning "lower," which may feel confusing by comparison to the modern concept of "upper and lower."
現・千葉県である安房国（房総半島先端）はともかくとして、上総国（房総半島中部）と下総国（房総半島根本部）の位置が「現代感覚から見て逆転している」のはこのためである。