Question about PVP Combat regarding Hit rating

So i have a level 50 twink rogue, and every now and then when im doing my world pvp ganking in Tanaris, when i sneak up behind someone to ambush, and as you all know you HAVE to be behind the target to ambush.

So why do i sometimes miss Ambush? When im behind the target? Is that a bug? or does it actually happen?

When when could you miss an attack when striking from behind? Its not a dodge, or a parry, its just a Miss. it happens 1/5 ambushes and totally wigs me out.

And it even seems i miss MOST of my melee attacks on players, and they are players between level 40-47... all 10 to 3 levels below me?

You need hit rating not to miss, expertise too to avoid dodge/parry/block.

So in PVP Combat, when attacking a player from BEHIND, with ambush, cheap shot, garrote, etc, i need HIT? something abundant in supply at low levels?

Yes, it's still needed.

Just get the hit/crit enchant on your offhand and gg