Does Nexus 7 has Bluetooth Smart technology? I've read here http://www.bluetooth.com/Pages/Bluetooth-Smart-Devices-List.aspx among other pages that it has this technology, but i'm trying to connect to a beacon with it and the app says that "Device does not have Bluetooth Low Energy".

Are there maybe any Nexus 7 devices without this kind of bluetooth or what is the problem?

The device is updated and the beacons are working because i can find them with other devices.
I assume you're referring to the 2012 version of the Nexus 7? Although the hardware is capable of supporting Bluetooth LE (as suggested by the link in your question), Google never certified it as such and so the stock ROM does not support it.

There is an answer to a similar question on stackoverflow which appears to suggest that CyanogenMod (and maybe other custom ROMs) has added support for Bluetooth LE on the 2012 Nexus 7.