iterating over ienumerable with linq2xml

I was surprised to see that not all of my elements in the following code are being iterated:

IEnumerable<XElement> dataStorageGroupElements = document.Descendants().Where(x => "Xms.Common.DataStorageGroup" == (string)x.Attribute("NodeType"));
int counter = 0;
foreach (XElement dataStorageGroupElement in dataStorageGroupElements)
{
    Console.WriteLine($"Counter={counter++}, NbElements={dataStorageGroupElements.Count()}");
    XElement newParent = GetNewParentForDataStorageGroup(dataStorageGroupElement);
    dataStorageGroupElement.Remove();
    newParent.Add(dataStorageGroupElement);
}

Output is:

Counter=0, NbElements=12
Counter=1, NbElements=12
Counter=2, NbElements=12
Counter=3, NbElements=12
Counter=4, NbElements=12
Counter=5, NbElements=12
Counter=6, NbElements=12

Changing the code to execute enumeration with ToArray() fixes this issue:

IEnumerable<XElement> dataStorageGroupElements = document.Descendants().Where(x => "Xms.Common.DataStorageGroup" == (string)x.Attribute("NodeType")).ToArray();

With the above code the counter is being increment to 11.

Why some elements are being skipped is described here. But why is NbElements always 12? Is it only evaluated the first time?