Maybe @Akimbo could shed some light on this question.
I would defer to @Duke Dive Medicine on the "why".
In practice, the Navy does very little decompression diving in normal operations. They occasionally get recovery and salvage ops that put them in planned decompression but they have decompression chambers on site and primarily do Sur-D-O2 — which is also the norm for commercial surface-supplied decompression diving.
Notice that Table 9 9, Air Decompression Table, combines in-water decompression on air, in-water decompression on air and oxygen, and surface decompression on oxygen (Sur-D-O2). Nearly all the Sur-D-O2 tables I have seen have their last in-water stop on O2 at 20'. The primary purpose is to get divers out of the water as quickly as possible for comfort and safety and to free up the dive station for the next team of divers.
Most of the company-proprietary Sur-D-O2 tables I have seen are closer to a treatment Table 5. They typically allow five minutes from leaving the 20' water stop to be at 60' on O2 in the chamber.