vertices with similar weights not moving at same speed

Question

As the above image shows, whenever I move the armature that controls the upper arm, the corresponding part of the t-shirt moves slower than it. This is regardless of whether or not they have the same weights.
The picture shows them at different weights, but I tested to see if the same issue arises when all weights are the same. The whole object is composed of one mesh the t-shirt and the arm are all in the same mesh. How would I prevent the arm from moving through the t-shirt?
(I know I can delete the vertices that are blocked by the t-shirt, but this same error persists even then for the remaining vertices that are slightly inside).

I am using blender v2.72b.

Answer 1

Upon further playing around with the mesh, I found out that I should do the following to get what I want:

• In edit mode, select the island of the cloth mesh, in this case, the t-shirt.
• Press P to separate the cloth, click by "Selection".
• Go to object mode, parent the rest of the mesh to the armature, with automatic weights.
• Then parent the cloth just separated to the armature too.
• Go to pose mode and move the body, the cloth will follow it.
• Go back to object mode, select the cloth then the body, and then join the two meshes, to get back the original mesh.
• Going back to pose mode and moving the armature around still shows that the cloth follows the body's motion.

That's it, this worked for me and I hope someone will also find it helpful. If anyone has a different way of approaching this, please share.