Taken from the steal skill description.
The formula for stealing is believed to be: DropRatio*(4 + 6*SkillLV + DEX - MonsterDEX)/100 where DropRatio is the percent chance of a particular item being dropped. If the result of the steal check is lower than the AdjustedDropRatio for an item, then it is possible that item will be stolen.
Level Description
1 Success Chance 10%
2 Success Chance 16%
3 Success Chance 22%
4 Success Chance 28%
5 Success Chance 34%
6 Success Chance 40%
7 Success Chance 46%
8 Success Chance 52%
9 Success Chance 58%
10 Success Chance 64%
On this server Myst Cases Drop cakes at a 24% chance and Myst Cases have 48 dex. So for you the inputs would be:
24*(4 + 6*10 + 85 - 48)/100
24*(4 + 60 + 85 - 48)/100
24*(64 + 85 - 48)/100
24*(149 - 48)/100
24*(101)/100
2424/100
24.24%= Result of Steal Check
Since this is Higher than the Drop Ratio of the cakes, I believe that is why you aren't having that much success with stealing. Most people stick with Steal lvl 8 as you still have a high chance to steal an item, and you won't risk not being able to steal an item due to the Steal Check being higher than the Drop Ration of the item you want.