bundle update --conservative devise does not update only "devise" gem

I recently tried to update devise to the latest version specified in the gemfile without affecting its dependencies.

gemfile.lock:

devise (3.5.3)
      bcrypt (~> 3.0)
      orm_adapter (~> 0.1)
      railties (>= 3.2.6, < 5)
      responders
      thread_safe (~> 0.1)
      warden (~> 1.2.3)
thread_safe (0.3.5)
warden (1.2.4)

After I run bundle update --conservative devise i got:

gemfile.lock:

devise (3.6.3)
      bcrypt (~> 3.0)
      orm_adapter (~> 0.1)
      railties (>= 3.2.6, < 5)
      responders
      thread_safe (~> 0.1)
      warden (~> 1.2.3)
thread_safe (0.3.6)    ---> changed from 0.3.5 to 0.3.6
warden (1.2.7)         ---> changed from 1.2.4 to 1.2.7

I am aware that only "devise" gem version changed and not the dependencies but the gems that changed their verions too like thread_safe and warder are also part of the dependencies of devise but out of the scope. What do they rapresent and why have changed their versions?