Yes, they're VPSs, but no, a VPS does not provide the level of isolation that you're imagining. RAM is easy to offer guarantees about, CPU can be (but you can often burst over your guaranteed allotment, which means there's often high variability in performance, even if you always get what you're technically paying for), and disk IO is very hard (and you'll notice cheap VPSes don't even try and promise you any specific amount of IOPS).
There's a million links discussing this, google "VPS noisy neighbor" or see, eg:
https://www.liquidweb.com/blog/why-aws-is-bad-for-small-orga...
Second, even if they allocate fixed resources to your VMs isolation is not that perfect, and you can see performance differences in things like memory access, storage speed, ...
It's really a measure of the load on the hardware by neighbouring accounts at the time of benchmarking.
A lot of these small virtual servers are packed into a single host. The actual performance you get depends a lot on how your neighbors are using the resources they share with you.