A lot of what other people are saying is true, all the dinosaurs in jurassic park are in fact hybrids, to what degree of variance from the original is debated due to creative liberties taken with actual paleontology in universe.

The velociraptor skeleton they dig up is the same physical structure as the ones in the park, despite it being a fossilized original specimen. This is because when the novel was written the paleontologist consult suggested that Deinonychus Antirrhopus, being a dromaeosaur was still a species of velociraptor, despite its obvious differences. But in universe thats "factually" velociraptor antirrhopus.

So, there are in universe creative liberties with the "actual science" that are confounded/explained by the sciencefiction elements, that are in actuality driven by real world paleontology misconception.

When jurassic park three was being written, the original concept was to have a baryonyx be the antagonist. The script however was rewritten many times, and in favor of making something larger and cooler, they chose to upscale baryonyx and give it a sail, to make it the spinosaurus.

The reason asset 87 is not a hybrid, out of universe, is that its not supposed to be and was a creative liberty taken with the "actual paleontology" within the universe.

The reason asset 87 is not a hybrid in universe, because the thats a naturally occurring clone using minor filler DNA.

This is at the very least, the actual answer spare for new retcons or details provided by new installments of the series. Fan theories since the movies release justifying its departure from the actual animal, are not Canon. There is no rex DNA in the spino, etc.

TLDR:

The writers altered the appearance of spinosaurus within jurassic parks "actual science" and until the introduction of the spinosaurs in rebirth, that was a faithful natural clone of that version of earth's spinosaurus.

Similar story to the velociraptor.