I was going for one of the star challenge achievement when this happened. Basically i got the last one i needed, but some how my system says i need 70/75 achievement left when it should have said 71/75. i look at the achievements and i see only 4 that i still dont have, but i look at my achievements on xbox.com and it shows that one achievement i know i have as being unearned! its the achievement for causing 10,000 points of damage with unique weapons. that was one of the first ones i got in the gunrunner dlc. why is this happening and how can i fix this.

again it shows it on my list on my 360, but the gamerscore doesnt count.