Some music has distinctive titles, like Romeo and Juliette or The Tree-Cornered Hat. More than one composer might use the same title, but so long as we specify whether we mean the Romeo and Juliette by Tchaikovsky or Prokofiev, or anyone else, there is no question of which piece we refer to.
Other music has form titles, like Sonata, Concerto, Symphony, etc. Not only have many composers used those titles over a long period of years, but many use them more than once. We keep them apart by numbering them, among other things. When we see or hear a reference to one of Beethoven’s nine symphonies, one of Brahms’ four, to name only two of many examples, we know exactly what piece is meant.
Not every sequence of numbers is that straightforward. The conductor of the orchestra I play in announced that he intended to play Dvořák’s Seventh Symphony on our next concert, but had second thoughts before the first rehearsal. He said he wanted to decide among that, Franck’s Symphony, and Dvořák’s Ninth. Everyone’s folder included parts for Dvořák symphonies number 2 and 5. Most of the orchestra was confused.
Antonin Dvořák composed five symphonies before he had any published. He issued one now known as his Sixth Symphony as his Symphony no. 1 and the present Seventh as no. 2. Before he wrote another symphony, he decided to publish one of the earlier ones (his Fifth) as no. 3, and so of course he published his last two symphonies as no. 4 and no. 5.
The thematic catalog of Dvořák’s complete works, now universally used as the definitive source of numbering and dating them, restored the four earlier, unpublished symphonies. After it appeared in 1955, it took a while for the new numbering to become accepted worldwide. Somehow it just didn’t seem right for the New World Symphony, known since its first publication as no. 5 to suddenly become no. 9. Now, it’s the older printed music and recordings that people find confusing.