Not all clones were created equal; some were created for various projects and elite programs per the then chancellor's request. One such batch of clones was created as a long term plan, in case all other methods to overthrow the republic failed. As such, they were modified to grow at normal rate, but went through the same rate of training as regular clones. Had they reached full maturity under the republic they would have been nigh unstoppable, as each one would have had twice the trianing of the most elite clone before stepping onto the battlefield.
However, this was not to pass. The Emperor's plans suceeded beyond belief, and the clone wars ended before they even had a chance to begin. Combine that with some of the members of the program going rogue, and the empiror's attempts to tie up loose ends, the program was terminated, and in true empire fashion, so was everyone affiliated with it.
Considering that no-one else survived the massacre or is aware of Naboo's survival, the exact details of his escape are known only to hi. Suffice it to say, he somehow survived the termination protocol and found himself trying to make a living in a world where his only skills are combat related. Fleeing to the outer rim, his skill at 'survival' quickly caught the attention of the Hutts, and he climbed the ranks as a Jabba bounty hunter.
He's now back with the empire under his well crafted and known bounty hunter alias, as the call for troops has created many openings in the imperial army. Because he has no problem with the empire, and in fact, his militaristic upbringing and harsh lifestyle makes him almost approve of the structure and laws the empire provides, which is in direct contrast with the lawlessness of Tatooine and the outer rim, he was glad to sign on. No, his problem is with Sheev himself, and he hopes to climb the ranks enough that he one day gets close enough to kill the man who eliminated his brothers.

To climb the ranks and destroy those within the system responsible for the execution of my brothers.
Also he grew up as a clone, fighting and the chain of command is in his blood

Contract 5: Works for the Empire
Oath 10: Revenge on all who wrought the death of my brothers

You see what is clearly a mandalorian male in front of you; if the tall, well built body with the all too familiar preportions didn't give it away, then the facial features most certainly did. However, looking at him, you don't notice the face for long, because his short, light blue hair and his deep red eyes have a habit of drawing people's attention away from his other features. If you managed to look past the hair you would notice an almost artistically placed scar on the right side of his face, starting near the bottom of his forehead and dragging down a little into his cheek. The rest of his face has the well chisseled features common amongst combat raised mandalorians. He seems the type of person most comfortable wearing full armour in the midst of combat, but right now he's simply wearing armoured clothing, with a combat knife on his belt and a heavy blaster slung across his shoulder. He steps into the ship quietly, in the manner of one who knows how to avoid being seen, and moves with a confidence that betrays his experience in combat.