because it was mixed whith another dinos dna. Or possibly because of inGen splicing in the DNA of modern animals to complete the geneome. InGen most likely added the DNA of a chameolon which gave the Carnotaruses this ability that they wouldn't have really had. This is similar to the larger size of the Velociraptors, the motion sensitive vision of the T-rex and Miasaurs, and the venom of the Dilophosaurus, Troodon, and Procompsognathids. keep asking questions.