Following the paradigm for [face=SPIonic]nou=soj[/face] as presented in Pharr 679, and remembering that substantives try to keep the accent in the same syllable:
N - word is accented on the penult. Penult is long AND ultima is short -> accent is circumflex.
G - ultima is long -> accent must be acute
D - same rule as G
A - same rule as N
V - same rule as N
N.A.V - same rule as G
G.D. - accent on antepenult (allowed only if ultima is short) must be acute
N.V. - the oi diphthong in ultima is considered short for determining accent. Hence same rule as N singular
G - same rule as G singular
D - same rule as G.D. dual for [face=SPIonic]nou/soisi[/face] or, for [face=SPIonic]nou/soij[/face], same rule as G singular
A - same rule as G singular
If your question really boils down to "why does the nominative singular take a circumflex rather than an acute?", that's harder. There is no requirement that an accented long penult followed by a short ultima must take a circumflex.
But, if you learn the form in the dictionary along with its accent, you can usually figure out how the accent changes as the word inflects.