"anno toto"/"annum totum"

[Carolus_Raeticus]

Salvete!

Today’s entry of my Latin calendar gave a short story about Ulysses. Easy enough, but something is bugging me:

Naufragio facto sociis amissis Ulixes natavit in insulam Aeam, ubi Calypso, Atlantis filia et nympha, agebat. Quae specie Ulixis capta > anno toto > eum retinuit neque a se dimittere voluit, donec Mercurius Iovis iussu denuntiavit nymphae, ut eum dimitteret. […]

The story is easy enough to understand. However, why did the author use anno toto instead of annum totum? Wouldn’t the accusative of duration be more correct considering that Calypso held him back for an entire year? Or do I miss some nuance? What exactly is the difference between doing something for one year and doing something within one year? Is it about emphasizing either the act or the duration?
Thanks for your help,

Carolus Raeticus.

[adrianus]

@A&G §424

.b. Duration of time is occasionally expressed by the Ablative:–
milites quinque horis proelium sustinuerant (Caes. Civ. 1.47), the men had sustained the fight five hours.
NOTE.–In this use the period of time is regarded as that within which the act is done, and it is only implied that the act lasted through the period.* Cf. inter annos quattuordecim (Caes. Gal. 1.36), for fourteen years.

• Furthermore, in the use of the ablative the author may want to avoid a suggestion that Calypso repeatedly tied up Ulysses to have her way with him.
  Porrò usu ablativi forsit negat auctor quod fieri potest Calypso Ulixem identidem alligasse ut cum eo sese oblectaret.