In Leviticus 25:29-30 what is the difference between the dwellings in verses 29,30 compared to the dwellings in verse 31?

My questions are why is the dwelling that is in the walled city able only to be redeemed during a one year period and that if its not it becomes established for ever and is not returned at the jubilee? Also, in verse 31 the dwellings in vi…