Composite rule v/s ruleset

Recently, I was working extensively on developing business rules in calculation manager. Wherever necessary I have an habit of using variables. In one instance, I wanted to run 2 rules on a save of a web-form. So what I did was created a new rule and added the required 2 rules. I was not expecting any issues as both these rule were running as expected. But when I compiled, it threw an error of duplicate declaration of variable....I used / defined same variables in both the forms.

I was initially surprised but later realized that it considers it as one rule and so the problem.

I then created a rule-set, added the required rules and worked.