There seems to be a bug with Splinter Cell: Conviction that stops the game unlocking the achievement for completing the game on the realistic difficulty setting. I completed it just now and noticed it didn't unlock, so I googled the unlock and it seems lots of people have had the issue. Here's a thread about it on Ubisofts forum: Splinter Cell Conviction Complete Single Player Campaign Achievement not unlocking. - Topic Powered by Social Strata

Apparently there is a way of unlocking it, but I can't be bothered to try it out just for 50g:

Originally Posted by iamthechaos on the ubisoft forums

*IMPORTANT FIX*
I was down on my luck, so I decided for the fun and possibility to play the last two levels and I complete the game again and pop, I got the achievement!

PLAY THE LAST TWO MISSIONS ON REALISTIC AND YOU WILL GET THE ACHIEVEMENT!

Hope this works for everyone and post results!

Mine worked fine . Even so i played the last mission again to get the spare achievement.

Did you have to sit through the credits to get it? I just skipped them.

No the Achievement pops up before credits come on.

There's been an update by Ubisoft:

Originally Posted by Some bloke at Ubisoft

Hi Guys,

We’re working hard on eliminating the bug that is preventing people from getting the “Realistic Difficulty” achievement for Splinter Cell Conviction (Complete Single Player Story on Realistic Difficulty).

While we can’t discuss timing right now we can confirm that this issue will be fixed in the next title update for SCC.

The fix will automatically detect if you’ve already finished the game on Realistic difficulty and award you with the missing achievement. This means you will not need to play through the single-player campaign a second time after the update comes out, it’ll all be handled automatically.

We apologise for any inconvenience this may have caused and thank you for your patience while we work to resolve this issue.