Accessing rule set arguments in a specific order ?

Hello Rules users,

I'm experimenting a very strange behaviour with the Rules module.

I've designed a rule set using two arguments : updated content and unchanged content.
My first rule tests a field wich is a kind of flag (0=false, 1=true). I want to know if the value has been modified from 0 to 1. So I have a condition :
If field was 0 in unchanged content AND field is 1 in updated content THEN some action
Well, at first it was not working. Result of first condition was oddly always false !
Then (after a lot of desperate testing) I discovered by chance that if I reverse the condition like this :
If field is 1 in updated content AND field was 0 in unchanged content THEN some action
it works perfectly.

Now I'd really like to understand what it means. Do arguments have to be accessed in the order they are furnished ? Does updated content have to be accessed before unchanged one ?

If somebody can provide an explanation, I'd be really grateful.

Thanks in advance.