NDMP data is not the same as normal backup data. NDMP data from a NetApp filer, is a UFS_DUMP, it's a particular format being used by NetApp. EMC NAS use a tar format, and other NAS vendors for NDMP use CPIO. NDMP i just a communication protocol, with the actual backup format varying by vendor. Good luck trying to restore your NDMP backup form NetApp on another vendors NAS. :(
Thus, additional processing must take place to open that data format, to do a verify.
Plain and simple, NDMP sucks, but is a necessary evil in some cases. In an ideal world, you would do a synthetic backup with an incremental forever model with target side dedupe. Then backups would be very fast, can be stored for longer is a smaller space, and ultimately it can be restored ANYWHERE. But alas, Synthetics in BackupExec have always been less than stellar on high file counts.