Here are 2 links with the details how the B-Series VMs are working:
https://singhkays.medium.com/understanding-the-azure-b-series-and-cpu-credits-cd6ad1c46094
https://www.danielstechblog.io/monitoring-azure-b-series-vm-banked-credits/
The variable facts I am referring to is the CPU performance. Depending on the burst mode/burst credits of an individual VM the CPU performance is variable.
https://learn.microsoft.com/de-de/azure/virtual-machines/sizes-b-series-burstable
The CPU performance is influencing everything in the VM ... OS, running applications, drivers, disk performance, network performance, ...
This might be the reason why you get the results during your tests.
At least that's my idea. Maybe someone from Microsoft has some more insights.
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Regards
Andreas Baumgarten