First, updates from Thunderbird 31.7.0 to Thunderbird 38.0.1 via Help > About Thunderbird are now active as I just updated one my machines. Important to note you must have 31.7.0 (released May 12th) in order to update to Thunderbird 38.0.1 as the machine only had Thunderbird 31.6.0 on it. After updating to 31.7.0 then it download the full 30+ MB update for Thunderbird 38.0.1 version.
In regards to the version numbering. Thunderbird 38 was suppose to be released on May 12th (same day as Firefox 38), but due to complications Mozilla could not release the full version of Thunderbird 38. Instead they released Thunderbird 31.7.0 which contained the security fixes which were suppose to be included in Thunderbird 38. For the record, there was not a Thunderbird 38.0 (or 38.0.0) release, instead their was an updated version of Thunderbird 38.0.0 released denoted with the ‘1’ in the 3rd place indicating a minor update to Thunderbird 38.0.
The next planned released will be Thunderbird 38.1.0 on June 30th (same day as Firefox 39).