I suspect that they are overprovisioning vms If you notice how it can seem laggy when logging into your vm at first? And since the swap partitions of the physical host are all SSD's it's not impossible that they are oversubscribing memory as well.
We do not oversubscribe memory and we do not provide SWAP space by default. Since its SSD based and memory is so cheap we recommend users size the machines according to their needs/requirements.