pana (class 16) = there is/are ... on/at/in (specific place)
kuna (class 17) = there is/are ... (in a non-specific place, large area, anywhere, just existence without mention of place)
mna (class 18) = there is/are ... in/inside (enclosed interior space)
Basically because it's "in the kitchen". There's some flexibility with these too.
Also, with the locative copula forms -po, -ko, and -mo, the class 16 form seems to be used to indicate existence without location a lot of the time as well. Wapo = They are there/here. / They exist. I think this is a bit less common with pana, with kuna taking this role.