Fixed Size VHD uses still high I/O on Physical Host?

So we did a performance test for our servers and found that one of them is using very high I/O.

The picture below shows that the server pretty much doing read/write very minute through out  a day.

This server doesn't do much beside hosting 3 hyper-V machines and all VM are using fixed size VHD.

After we did a individual test on each VM and found out one of the VM(SQL server) has high I/O.

I thought that if it's a fixed size VHD, it shouldn't affect the I/O performance on the physical host because it already allocated the space while the VHD is created.

Is this not true? please advise?