When transferring items through the CSTS system they are moved based on the relative distances to the destination.
Should you during this time send MORE of the SAME resource to the SAME location the original stack will meet up with the newly incoming one.
This effectively resets the time on the original stack to that of the new one. This is not a Bug, but is a design system to stop players managing to replicate resources as was tried in the early stages of Beta.
So sending the same type of resource to the same location twice they system will reset both deliveries to the later time frame.