By default, vSphere provides a mechanism that creates scheduling queues for every virtual machine file. Each file, for example .vmdk, gets its own bandwidth controls.
This mechanism ensures that I/O for a particular virtual machine file goes into its own separate queue and avoids interfering with I/Os from other files.
This capability is enabled by default. To turn it off, adjust the VMkernel.Boot.isPerFileSchedModelActive parameter in the advanced system settings.