The difference in them besides the mg of medicine is the coating on the tablet.
They are designed to release the medicine at a specific ph level, which happens inside your intestines at different locations. I dunno exactly which one releases where, that is something to ask your doctor or pharmacist, who will gladly give you a ten minute discussion, as you have hit the nail on the head as far as what they consider to be interesting.
Try this out. Let one of them sit inside of your mouth for several minutes, spit it back out, and look at the tablet, notice that it did not dissolve one bit. ;)
Humira 2 times a month.
Lialda 2 X day
Imuran 200mg day
Remeron 30mg day