I'm troubleshooting this site for someone else and that's what I initially thought, but they said they had a user group where the edit rights were removed but allowed them to add items to the list (it's not a library).
From further digging since post I see there are inherited rights but not sure exactly what and the user group the user is in definitely does not have edit rights so I'm thinking their new user group isn't actually doing anything at all
I just wanted to check if this was a genuine issue someone may have come across before so could rule out some things
Never mind I'll just keep digging and suggest stripping back to basics..