bundle update isn't updating to most recent available version

I'm not sure what the problem is, but let me present my observations:

Started with no paperclip version set and no dependencies on paperclip: 
$ cat Gemfile | grep paperclip 
gem 'paperclip' 
$ bundle update paperclip 
... 
$ cat Gemfile.lock | grep paperclip 
    paperclip (3.2.1) 
  paperclip

Updated Gemfile to explicitly declare the current paperclip version: 
$ cat Gemfile | grep paperclip 
gem 'paperclip', '3.4.0' 
$ bundle update paperclip 
... 
$ cat Gemfile.lock | grep paperclip 
    paperclip (3.4.0) 
  paperclip (= 3.4.0)