Why did the phrase "upon the earth" appear in the verse {And the servants of the Most Merciful are those who walk upon the earth humbly} while "in the earth" appeared in all other verses?

The exegetes did not clarify the wisdom behind the Almighty’s saying: "walk upon the earth humbly," even though walking is mentioned in other verses as being "on the earth." Perhaps the meaning in all instances is the same, and this falls under the category of the diversity of Quranic styles.