But in most lines (in fact in 88 out of the 93 lines of Catullus 63) the 13th element is resolved into two short syllables, leading to a run of short syllables at the end of the verse.
In most cases (specifically, 88 out of 93 lines in Catullus 63), the 13th element is reduced to just two short syllables, causing a string of short syllables to appear at the end of the poem.