I see what you mean. 38.0.5 was a special marketing release to promote Pocket and new Hello features. You can see on Mozilla ftp server that the version numbers of the 38.0 patch releases jump from 38.0.1 to 38.0.5 (leaving room for any emergency patch releases of 38.0.1):

https://ftp.mozilla.org/pub/firefox/releases/

38.0 was released on 2015-05-12. 38.0.5 then sat in the Beta channel for two weeks of testing and then it was released on 2015-06-02:

https://wiki.mozilla.org/RapidRelease/Calendar#Past_branch_d...