Why is there an arrow next to the letters on the middle key, and "Enter" on the far right next to the numbers? Is there a difference?

Yes, one is the return button and the other is the Enter button;-)

It’s still from the typewriter eras. With the return button you have come back to the beginning of the line and with the Enter button you have completed the line and have come to the left edge of the sheet in the next line.

Nowadays, the buttons make the same function: they close the current line and then jump to the beginning of the next line.

https://de.wikipedia.org/wiki/Input button

https://de.wikipedia.org/wiki/Wagenrücklauf