Late response, but if you're still playing on Factorio 1.1, you should check the documentation for this mod. If I recall correctly, it states somewhere (I forgot where) that the receiving teleporters have hidden buffers that can store up to 25% of the total capacity. This way they can a receive a full 100% load while it still has ~25% capacity filled, and stores the extra in the hidden buffer. As it empties it will replenish its main inventory from this hidden buffer. This is to ensure constant throughput without running dry, given various delays in the teleport cycle. Likely that is what is occurring in your example. Hope that helps.
