Using reflection to add to a collection in a static class results in multiple static classes receiving the value

I have 2 static classes that hold collections of items that are each used for separate areas in the system. Here is the issue I am having:

When I use reflection to add an item of type X to one of the static classes, which each hold their own collection of type X, the item gets added to the collection of each static class, when I only want it to be added to one of my static classes.

Before the following lines are executed:

//FirstStaticClass.TypedObjects.Count: 0;
//SecondStaticClass.TypedObjects.Count: 0;

Retrieving the PropertyInfo of my target static class:

var collectionPropertyInfo = typeof(FirstStaticClass).GetProperties().Single(x=> x.GetCustomeAttributes.Any(y=> y.GetType() == typeof(AttributeIAmLookingFor));

Adding my object to the collection retrieved above:

collectionPropertyInfo.GetValue(null).GetType().GetMethod("Add").Invoke(collectionPropertyInfo.GetValue(null), new[] {objectToAdd});

Immediately after the above line executes, I can inspect each of my static classes and they each contain the item:

//FirstStaticClass.TypedObjects.Count: 1;
//SecondStaticClass.TypedObjects.Count: 1;

In the above example, my static classes each have a collection of the same type, and are named the same. I thought that the reflection was finding all properties with the same name, so I tried changing it. However, even with a different name, the collection on the SecondStaticClass would contain the added object:

//FirstStaticClass.TypedObjects.Count: 1;
//SecondStaticClass.DifferentNameSameType.Count: 1;

I searched for a while without being able to find anything relating to my specific issue. Any help would be appreciated.