That top line are the names of the properties in the Datastore object that are displayed by default.
That doesn't explain the 5th line.
But is that a blank line at the end ?
Can you do
Get-Cluster $clustername | Get-VMHost | Select -First 1 | Get-Datastore | Measure-Object | Select -ExpandProperty Count
That should say how many datastores are returned for the 1st host in the cluster.
You could also do something similar for the 2nd host in the cluster.