Hi Martin,
Thank you ever so much for the question, it's a good one!
The Bail Bond (BB) limit on a Thai insurance policy is not strictly determined by claims history, NCB, or driving record. Instead, insurers apply their own actuarial modelling, which often factor in the type of vehicle, perceived likelihood of an incident requiring bail, and their specific overall portfolio performance across similar risks.
Motorbikes often attract higher scrutiny from police and carry a higher perceived risk of serious incidents (accidents involving injury, fatality, or alcohol, for example). This is regardless of your personal history. Therefore, insurers often apply a higher BB limit automatically to motorbikes rather than cars, due to portfolio experience and that can evolve and change over time and any prevailing legal or regulatory notes on this can also have a bearing.
I hope this helps, but what we'll do as well, as you mentioned MSIG, is reach out to them (we won't disclose your name/ID to them) to get their written response to this too. We are one of their larger brokers so I am sure they will help us out with their specific insights.