The suffix may have been diminutive, or the word was originally an adjective, like "with a shaft, with a handle".
No good examples of *-tEdA adjectives in Proto-Finnic, -teA adjectives in Finnish are all quite young.
Likely parallel: *musta < *mustë-ta (no **mustta for phonotactic reasons), from *muste(“dirt? dirty?”), cognates could include Northern Samimostot (< *mostōtēk, if not borrowed from Finnic to Samic)
varsta from vartta by influence from varsi; still shows the derivation was at least somewhat familiar. There was pressure to change the word because of its homonymy with the partitive singular of varsi. varsta is only found in some Finnish dialects (Tavastian, South Ostrobothnian), so clearly secondary.
Same might apply to kartta(“hard crust, hard dirt”) ~ karsta, but distribution does not align up (karsta has a much wider distribution), so might be secondary.
