packages are the same for FreeBSD 10 and 11?

Packages I can install in FreeBSD 11 via pkg install ... are exactly the same (same version) I can install in FreeBSD 10 ?
Package versions, yes. But packages are specifically built for 10 or 11. So you will have a packageA-1.0 for 10.x and a packageA-1.0 for 11. The packages are the same version but are built specifically for a version of the OS.

You cannot install 11.x packages on 10.x but you can install 10.x packages on 11.x by using misc/compat10x for example.

Note that all versions on all supported architectures use the exact same ports tree. So in essence everything will be available for all version. There are a few exceptions though for ports/packages that use specific features that aren't available on older FreeBSD versions. But those are far and few in between.