Genetics?

They may be growing exactly the same, but every seed/plant has its own unique genetics (genotype), leading to many variations in plant expression (phenotype). The weird one knows what it is doing, it is just weird and an individual. Not much you can really do anyhow, just keep loving it and treating it the same as the other one. She may be a bit slower and later than the other one, but it will get there eventually. Plants are like children, even though they have the same parents (genotype) and the same up bringing, each child is different and unique (phenotype), just like it is with these two plants. Each seed is its own unique genetic "parcel", made organically and not a replicant made to an exact recipe in a factory. This difference is normal in plants and is known as "genetic variation" and is something breeders look for, that unique super phenotype that is clearly superior to its brother and sisters, that one magical wonder plant that is awesome in every way and a definite "keeper". Unfortunately your weirdo is probably not one of them, but you never know, she may grow flowers that are beyond belief! Cheers, Organoman.