User blog comment:SilenceInTheLibrary/Annie's Future/@comment-4361160-20130826214228/@comment-7418318-20130912232629

He can harden his whole body over very long periods of time, where Annie can just harden small areas for a very short time.

Most likely she copied it from him, but can't do it so well because it's not her natural talent.

And even so there are differences. On Reiner the armoring is opaque and metallic in appearance (and in the anime the cannonball that hit him made a metallic sound too), while Annie's armor seems to be translucent crystals, just like the crystal she encased her Human body in.