Why can't the system distinguish the difference from "2nd Ave " "2nd ave A" " 2nd ave B"? it makes no sense to me. usually the system will place it as 2nd ave, but if you go into "details" on the diad you will see the "A" where it will usually put the apt #.
This is about how the shipper (including UPS Stores and our own counter personnel) put it in the system. The system would rarely have a chance to "distinguish" this, because it doesn't matter to the system in most cases.
It can be as simple as how you type in the address in the address line on ups.com if you were to print a P/P label. As an example: the address is to "455 imaginary street" and there are units A through L here. If the person putting in the info (just a customer printing their own label in this case) just goes ahead and puts "455 imaginary street apt C" in the address 1 line, it will not give you details in your board of "C". If, however, they put "C" under the Apt slot, it should show accordingly.
Hope that helps, it's only one of a plethora of possible scenarios and circumstances.