Yeah this is an auto-numbered thing when the sandbox is provisioned. So probably you did have an SB1 provisioned at some point (maybe during implementation) that you stopped paying for and thus got taken away by NetSuite and then asked for another one, or you started with 3 sandboxes and decided you only needed 2 later. If it makes you feel any better our current sandboxes are SB1 and SB3, which I feel like is even worse 🤣