Why do internal IDs sometimes increment by 1, but other times by 100?

It does depend on the record type, but some things like an invoice, sales order and customer payment all share the same base object in NetSuite of a transaction. By adding a new invoice it creates a new transaction, incrementing the internal id of the transaction object which is the base for the others, Meaning that the next sales order generated uses the next transaction ID rather than a sequential sales order internal id

Thanks Kieran. Yeah, I have seen that with the transaction record type. But I'm even seeing it on custom records (that have their own int ID sequence).

all transactions come from the same table. all custom records also come from the same table.

I think you're talking about something else, as that would imply that two different custom record types couldn't have records with the same internal id, which isn't the case.

no, im talking about custom records as well.