Because Rock Symphony BLUE SONNET is actually an image album, not a soundtrack album. They are not related to the OVA.
The Japanese Wikipedia categorized it as an image album.
- Rock Symphony BLUE SONNET (1982)
- Rock Symphony BLUE SONNET II
- BLUE SONNET III Ballade
- BLUE SONNET IV DESERT CITY
According to a Japanese blog article by joelover, for image albums, quoting a book by Joe Hisashi,
The image album precedes the soundtrack. The clues are the illustrations and words that convey the image of the work. The image album is created based on these images.
(Quoted from I am - Faraway Music Road - by Joe Hisaishi, 1992, p. 37)
Compared to the soundtrack,
A soundtrack often refers to the actual music used in a play. Recently, the term OST (Original Sound Track) is also used to indicate that it is the music accompanying a play.
So, Rock Symphony BLUE SONNET and its subsequent image albums are original audio works made specifically to capture the image of the world of Blue Sonnet series.
In conclusion, the OVA was made to adapt the manga series, but the image albums were made to expand the story instead. They are not related to each other, both production-wise and purpose-wise.
For direct comparison, taken from Joe Hisashi's own works for Ghibli production, Princess Mononoke has both image album and soundtrack album: