This is a bit like someone being mystified that their arrival time at a destination across the city is not repeatable to within plus-minus a minute.

There are traffic lights on the way! Other cars! Weather! Etc...

I've heard that Google's internal servers (not GCP!) use special features of the Intel Xeon processors to logically partition the CPU caches. This enables non-prod workloads to coexist with prod workloads with a minimal risk of cache trashing of the prod workload. IBM mainframes go further, splitting at the hardware level, with dedicated expansion slots and the like.

You can't reasonably expect 4-core virtual machines to behave identically to within 5% on a shared platform! That tiny little VM is probably shoulder-to-shoulder with 6 or 7 other tenants on a 28 or 32 core processor. The host itself is likely dual-socket, and some other VMs sizes may be present, so up to 60 other VMs running on the same host. All sharing memory, network, disk, etc...

The original article was also a network test. Shared fabrics aren't going to return 100% consistent results either. For that, you'd need a crossover cable.