Request For Assistance: D-OA Seepost Postcard With Odd Routing

I recently picked-up this postcard in an auction lot containing a handful of cards, and I'm having difficulty deciphering the sequence of events.

This postcard was first cancelled with an Ost-Afrikanische Zweiglinie "I" postmark on 6 August 1899.  According to the ArGe catalog, this would have been the steamer Peters on the south-bound Zweigline, though research in this area is still ongoing.

The text of the postcard then indicates it was written while on board the steamer Reichstag while in Beira, Mozambique, on 7 August 1899, a day AFTER it was postmarked on the Zweiglinie.

The card then received postmarks from the Reichstag using the Hauptlinie "b" canceller on 8 August 1899.

Also, if I'm reading the rest of the text correctly, it states that "the postcards from Mozambique were sent via the Kaiser", and I assume that's a reference to other cards and not this one, since the Kaiser used the Hauptlinie "d" postmark at this time.

I'm just confused as to how the card received Zweiglinie postmarks the day before it was supposedly written while on board a different ship on the Hauptlinie – unless, of course, the sender hand carried it from one ship to the other, writing the text in between postmarks.