Different instances of the same family with the same price

This isn't just for hpc7a but also hpc7g as well. Looking at the price tag, and as you get the same amount of memory, it seems you are paying for the whole server (96xlarge) but can provision less cores. Traditionally you would do this to optimise your license costs but there can be other use-cases in HPC as well* You can read more from below blog that covers hpc7g (but I assume most of it applicable for hpc7a)

"By offering instance sizes that already have the right cores turned off, you’ll be able to maximize the performance of your code with less work."

https://aws.amazon.com/blogs/hpc/application-deep-dive-into-the-graviton3e-based-amazon-ec2-hpc7g-instance/