The rightsizing tool will not make recommendations if the hourly rate for a VM is lower than other sizes due to RI application.  This can be validated by assessing the pricing and asset pages for Azure VMs.

For more information on Azure pricing, see the following resources:

https://azure.microsoft.com/en-us/pricing/details/virtual-machines/linux/

https://azure.microsoft.com/en-us/pricing/details/virtual-machines/windows/

https://azure.microsoft.com/en-us/pricing/calculator/