Why have Behavior Packs if you can not earn achievements

The Invincible Pack will only block Achievements if you activate them. You could buy the pack and as long as you don't turn them on in your phone, you can still earn Achievements.

Multiple achievements in Forza 7 that noted Gem earned on screen that have shown up.

The gems that show up when you earn an achievement is just to note that it is a rare achievement. It doesn't have anything to do with gems earned through the Rewards program. In order to earn gems you either have to complete a challenge (which are listed
on the Missions page of the rewards site), complete offers, and you can can earn gems for every 1,000 gamerscore you earn and/or reaching new achievement tiers.

I have already recieved some for various achievements that have been done on there so why did I get those then?

Well, You earned an achievement at some point......Why delete it??? You can delete ZERO achievement games from your history but, once you reach a milestone in game, it's saved as being just that.... An achievement. Why lose what you earned?????? GOOD
LUCK!!!!!!

[quote user="noboundz"]

You got "That's a Knife," which is indeed a Halo: Reach achievement, not a Halo: CE Anniv. one. Perhaps you can obtain Reach achievements through CE's firefight mode?

[/quote]

Yes you can as you are playing Halo: Reach when playing on the Anniversary Map Pack.

Any Achievements earned while playing the Halo: Combat Evolved Anniversary Campaign are for
Halo: Combat Evolved Anniversary. Any Achievements earned while playing on the Anniversary Map Pack are
Halo: Reach Achievements, and since it's Halo: Reach Multiplayer any standard (non-map specific)
Halo: Reach Multiplayer Achievements can also be earned.

if you have your gamer score reset due to cheating, you deserve it and should not have cheated in the first place. Also why should you be allowed to earn they again, if you could not be bothered to play the game in the right way in the first place. Anyway
you can still earn achievements on games that you didnt get any achievements on, and also new games that are now being released you will be able to earn achievements.