The SOP with a 48 is to bring it as close as possible to right under the center of the vessel to detonate and break its back. Like this (at 3:15 +)
Now, the video above is of a sinkex. The target is not moving nor fighting back, unlike the Iranian frigate in the present case. Add to that the fact that the Iranian frigate is not as big a target as the Amphib in the sinkex and this attack was the first actual operational torpedo firing by a US SSN, and in the end, one miss and a second one close enough for a kill is pretty reasonable.