I did a push to Heroku, modified a little code (but no gem and no asset changes) and then pushed again.
The second time, the deploy messages indicated all the gem were 'installed' instead of saying 'using' the prior gems.
Why might Heroku re-install all the gems when none changed?
Heroku will update all gems if your version of ruby has changed.
That is because some can compile C binaries, which would then not be compatible with earlier versions of ruby, and therefore need to be reinstalled.