Why does wily already have -proposed pocket while still being in development

[Pascal De Vuyst]

Why does wily (Ubuntu 15.10) already have -proposed pocket at this moment, e.g. latest Firefox 39 is in wily-proposed while wily still has Firefox 38 and trusty, utopic and vivid -updates pocket already has Firefox 39. I would expect Firefox 39 to be in wily directly instead of wily-proposed since wily is still in development and doesn't need StableReleaseUpdates yet. Shouldn't the -proposed pocket be activated on FeatureFreeze (more than a month from now) in the wily release schedule: https://wiki.ubuntu.com/WilyWerewolf/ReleaseSchedule?

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: firefox 39.0+build5-0ubuntu1
ProcVersionSignature: Ubuntu 4.0.0-4.7-generic 4.0.7
Uname: Linux 4.0.0-4-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.17.3-0ubuntu4
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: pascal 1440 F.... pulseaudio
BuildID: 20150629110007
Channel: Unavailable
CurrentDesktop: Unity
Date: Fri Jul 17 13:33:18 2015
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2015-07-13 (3 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150713)
IpRoute:
 default via 10.0.2.2 dev enp0s3 proto static metric 100
 10.0.2.0/24 dev enp0s3 proto kernel scope link metric 100
 169.254.0.0/16 dev enp0s3 scope link metric 1000
IwConfig:
 enp0s3 no wireless extensions.

 lo no wireless extensions.
Locales: extensions.sqlite corrupt or missing
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=39.0/20150629110007
RfKill:

RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/01/2006
dmi.bios.vendor: innotek GmbH
dmi.bios.version: VirtualBox
dmi.board.name: VirtualBox
dmi.board.vendor: Oracle Corporation
dmi.board.version: 1.2
dmi.chassis.type: 1
dmi.chassis.vendor: Oracle Corporation
dmi.modalias: dmi:bvninnotekGmbH:bvrVirtualBox:bd12/01/2006:svninnotekGmbH:pnVirtualBox:pvr1.2:rvnOracleCorporation:rnVirtualBox:rvr1.2:cvnOracleCorporation:ct1:cvr:
dmi.product.name: VirtualBox
dmi.product.version: 1.2
dmi.sys.vendor: innotek GmbH

[Pascal De Vuyst (pascal-devuyst)]

$ LANG=C apt-cache policy firefox
firefox:
  Installed: 39.0+build5-0ubuntu1
  Candidate: 39.0+build5-0ubuntu1
  Version table:
 *** 39.0+build5-0ubuntu1 0
        500 http://be.archive.ubuntu.com/ubuntu/ wily-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     38.0+build3-0ubuntu2 0
        500 http://be.archive.ubuntu.com/ubuntu/ wily/main amd64 Packages

[Adolfo Jayme Barrientos (fitojb)]

-proposed does not equal SRU. It is a staging pocket.