I guess you have to read the "USB Storage App" as the USB storage used by the app itself. So the code of the application.
If your application would for instance start generating cache information, saved pictures etcetera. And if it would save that information on your Storage Card, then the information will be showed under Storage Card.
So under "App" and "Storage Card App" is only the information and code of the application itself, not the information or data it generates. So in most case these 2 numbers will not change over time. And then under "Data" and "Storage Card" information generated by the application can be found. Either on the Storage Card or the internal memory (depending on the application I guess). This is the information managed by the application (local databases etc), so this will probably change over time, depending on the application in question of course.
This is my interpretation of those 4 terms. And if I check for some applications I use, it looks like my interpretation is correct. It is certainly correct for the applications I created myself, so for these apps I know where what information is saved on the phone.