Okay.
I'm a uber master illusionist. 100 in deceptive awesomeness. Plus 70+ in sneak.
I'm heading from Winterhold south toward a cave somewhere in those mountains for a quest. I'm being slow and careful, because I've been gutted by random wildlife along this road in the past.
Sneaking up over a ridge... uh-oh! There's a sabre cat!
I wave my hands and muffle my feet. Another wave and *poof* I'm invisible!
I'm going to make my way past this little basta--
Growl!
It turns and in two pounces whacks my head off.
I reload.
Well, okay... I know where the sabre cat is, so I make a wide path around that.
Wave wave. I muffle and go invisible again. Creep wide around, through the grass... creep creep....
And guess what comes running up out of the blue?
A bandit, demanding money.
What the hells?
Okay. I can rationalize the cat. He smelled me or something. When it happens with Brotherhood Assassins, I just assume they have some incredible assassin hearing.
But random, crappy bandits? Demanding money?
Sure, it was daylight. But I'm one of the most talented illusionists in the land. 100+, baby! I can't muffle/invis my way past some scrub mugger hiding in the weeds?
Does anyone find muffle/invisible handy? Unless I'm sneaking as well, I find it to be absolutely useless.

If your sneaking isn't really good you will get caught. You have to complement sneaking with magic dude.

I believe casting the spell makes noise, so if you cast it too close to an enemy they will know you're there. Try casting it when you know there aren't any enemies within earshot and see if you remain hidden when approaching an enemy.

There's quite casting perk.

Yeah, I know, but that wasn't the problem.
I cast. I'm watching the cat. Start moving. Pounce.
The robber didn't jump me until almost the expiration of the spell, so I'm pretty sure I had enough time there, as well.

Thanks for that tip! Next time I'll try some magic, like muffle or invisibility, dude.

Since the OP didn't mention he had the perk, I assumed he didn't. If he does, then I'm not sure what is causing the detection. I have 100 sneak and casting invisibility near enemies still alerts them, but even though I don't have the quiet casting perk I can still remain hidden as long as I cast is far enough away from anything aggressive.

In that case, I'm not really sure =/ Are you running any mods that might affect detection?

Edit: Didn't mean to double post, I thought I was editing my last post.