Windows 10 is set for general availability of July 29th, 2015. So the Windows 10 RTM build sign-off date seems about right, and it takes some times for OEMs, manufacturers, partners and retailers to move inventory through the channels to make them available to consumers on the Windows 10 launch date itself.
In fact, there are speculated build numbers for Widows 10 RTM candidate as follow (not confirmed though):
10.0.10176.0.winmain.150705-1425 (10.0.10176.16384.150705-1425 or 10176.0.150705-1425)
Above is from the winmain branch, which is typically the branch which is released as stable RTM version. Or another possibility (not confirmed too):
Above is from the th1 branch, which may mean the “threshold”, the codename for Windows 10 during initial development period. If this is true, another branch, th2, is said to be for “Redstone” release.
All builds above was compiled on July 5th, 2015.
It’s important for a build of Windows 10 to be set as RTM milestone so that it can serve as a stable product that sells to consumers. But Windows 10 is not yet done, as development continued even after RTM. Windows Insider program, where enthusiasts serve as beta testers for Microsoft is continuing for unspecific duration (thus you can get free Windows 10 by joining as Insider), so in essence, Windows 10 is designed to be a perpetual beta, constantly being updated and improved, though Microsoft will select a milestone, think SP1 or 10.1 or as such, such as rumored Redstone which scheduled in 2016 to deliver to non-Windows Insiders general public.
Once Windows 10 RTM is shipped to system builders, it’s highly likely that the build will leak to Internet, allowing Windows 10 lovers to get early access to RTM build.