Note that the Kalevala is not the original source for the Sampo (or anything else). In the early 1800s, the author of Kalevala, Elias Lönnrot, along with others travelled around Finland and Karelia writing down local folk lore. He then composed the Kalevala by putting various folk stories and songs together in a single more or less consistent epic. In the process he had to pick and choose which stories to use, which versions of each story to use (there often were many different songs about the same story), where to put them in a chronological order and how to tie them together in a single narrative (including adding his own material or modifying the existing songs).
You are correct that the description of the Sampo given in Kalevala is pretty clear. However, that's simply because that's how Lönnrot chose to represent it. Other stories have different and conflicting desriptions.
For example, a common story about Sampo is the theft of Sampo. In the Kalevala version (songs 42 and 43 in the Finnish 1849 edition) Väinämöinen, Ilmarinen and Lemminkäinen sail to the North to steal the Sampo. Louhi, the mistress of the North, pursues them. She turns herself into a giant eagle and grabs the Sampo back from the heroes, but Väinämöinen hits her claws with an oar. The Sampo falls into the sea and is broken. The pieces bring wealth to the lands they drift to.
The following short story collected in 1817 (SKVR VII5 Metsäsuomalaiset 10) gives a different version:
Vanha Väjnämöjnen, ja nuori Jompajnen, lähättiin pohjan muaale Sammasta hakemaan. Sieltä suatiin Sammas kiin. Lähättiin merällä. Sano nuori Jompajnen vanha Väjnämöjsälla: alutto jo virsis. Viel on virsinen varrastajn kujn pohjolan muaan uunit kuumotta. Länsi pa Sammas pilvee. Löj nuori Jompajnen miekalla kax varvasta Sammalta pojki; yxi länsi meree, toinen suatiin muaale. Joka länsi meree siite tuli suolat meree. Joka suatiin muaale, siite tuli hejnet muaale. Kujn ojs useet suaanut, nin ojs vilja tullut ilman kylvämättä. Obs! pohjan maan portit näkö tuin uuuin kuumotta.
Old Väinämöinen, and young Jompainen, went to north-land to bring back Sampo. There was Sampo caught. Embarked to the sea. Said young Jompainen to old Väinämöinen: begin your song already. It is early for song when the ovens of north-land still feel warm. The sampo flew to the cloud. Young Jompainen stroke with a sword two toes off Sampo; one flew to the sea, other got the ground. What flew to the sea, it became the salt in the sea. What the ground got, it became the hay. If more was obtained, crops would have come without planting. [the last sentence I don't understand well enough to translate]
Here the Sampo seems to be some kind of a bird. Perhaps the giant Eagle that Louhi turns herself into in other stories.
For another example, SKVR VII1 679 (too long for me to translate the whole song) tells a variation of the story where Väinämöinen, Ilmarinen and Joukahainen travel to the North to steal cattle or quarry rather than the Sampo. Instead the Sampo is actually the ship that they sail on.
Iki vanha Väinämöinen Ancient old Väinämöinen
Meni riista riihen luokse, Went to quarry barn [/kiln?],
Oluella ukset voiti, With beer greased the doors,
Kaljalla saranat kasto, With beer wetted the hinges
Jott ei ulvo Pohjan Ukset, So wouldn't howl North's doors,
Nau'u Hiitolan saranat. meow the hinges of Hiitola.
Latjaeli laivan täyen, Loaded a full ship,
Saatto suuren sammon täy'en, Guided a grand Sampo full,
Laitto laivansa merelle, Put his ship to sea,
Saatto sammon lainehille. Guided Sampo to the waves.
From the song it seems like the heroes already had the ship-Sampo, but there is one part that might suggest that they stole it as well:
Virkki nuori Joukamoinen: Said young Joukamoinen [/Joukahainen]
"Laula, vanha Väinämöinen, "Sing, old Väinämöinen,
Hyreksi, hyvä sukunen, Hum, good kin,
Pohjolassa käytyäsi, Having been to North,
Hyvän sammen saatuasi!" Having gained a good Sampo!"
It's not clear why Joukahainen says that they gained a "good Sampo" if they already had the ship. However Louhi doesn't mention anything about having lost a ship when she notices the theft:
Tuosta Pohjolan emäntä Then the mistress of North
Juoksi riista riihen luokse Ran to the quarry barn
Kartanoa katsomahan: Too see the estate:
Riista kaikki pois kadonna. All quarry had disappeared.
Katso karjansa katoavan, Sees her cattle disappearing,
Alenevan arviohon. lowering the estimate. [not sure how to translate this line]
The singer of this song explained themselves that the Sampo is indeed a ship. Although if you ignore their explanation and only look at the words, you might be able to interpret it so that the Sampo doesn't actually refer to the ship, but to the stolen quarry.