The tune ‘Ellacombe’ is used to sing multiple hymns: ‘Hail to the Lord’s Anointed’ (for Advent), ‘Hosanna, Loud Hosanna’ (for Palm Sunday), ‘The Day of Resurrection’ (for Easter) and ‘I Sing the Mighty Power of God’.
This is probably a good thing if the texts fit together in their broad themes while also retaining their different identities. It’s a bit like how a musically informed listener of the Kyrie in Mozart’s Requiem in D Minor, knowing the fugue subject’s drawn from Handel’s Messiah, can pray the Kyrie (translated ‘Lord, have mercy; Christ, have mercy; Lord, have mercy’ in English) with reference to ‘And With His Stripes We Are Healed’:
The resulting unity comes not from the authors who penned the words but from the compilers who decide to weave texts together as interrelated but not monolithic texts. This results in a collection where each hymn interacts with the rest rather than being squeezed with the others into one entity with a single narrative and logical flow. You might compare it to canonical unity in individual diversity.