This hymn arrangement includes different keys:

C major and D major.

Each major keys has a total of 10 arrangements.

Why in many different keys:
This series is design for hymn lovers to enjoy playing the hymns they love. Some player loves to sing along with the lyrics.
For the singing purpose: For the human vocal, we can only reach certain pitch. In general, most of human (not includes professional trained or gifted singers) can only reach high E. The high E is E5 on the piano keys. However, some people have hard time to sing over high C which is C5 on piano keys.
For playing purpose: C major is the easiest key to play with. There is no keys signatures. Only few or no black keys involve in C major. Therefore,  you will see C major on all hymn arrangements. However, in some hymns, it is too hard for player to sing along with C major. Because some melody has notes higher than high C (C5).
You can choose the suitable keys for play and sing along together. If you want to play only, C major will your best choice.

Additional Music theory and technique:

As a Deer 01: Melody only

As a Deer 02: LH one note accompany

As a Deer 03: LH simple chords accompany

As a Deer 04: LH root chords accompany

As a Deer 05: LH inverted chords

As a Deer 06: LH simple broken chords

As a Deer 07: LH broken inverted chords

As a Deer 08: LH broken inverted chords accompany style 1