Sometimes there are just oddball blooms. Regular use of hormone products can also cause bloom malformations.
Has this bloomed normally before? I got an out of bloom plant last year that recently started to bloom, and all the blooms so far have an extra part - I suspect this one may be a bit of an oddball plant
I also had some Miltonia blooms last year that had some blooms with missing or underdeveloped petals - this plant grows outside, and we had some unusually cold nights when the buds were developing, which I think was the cause (plant had all normal blooms previously) .