Same Description, Same Rule
Rules should not surprise players. More specifically, if you describe a situation to the players and then describe the rules or modifiers that will apply because of the situation, the players should not go “whaaaa?”
If they are surprised it's either because you specified an odd mechanic (a will save to resist poison) or a really implausible modifier (-6 to hit for using a table leg as an impromptu weapon).
As GM you might think a rule or modifier fits even if the players don't agree. It's bound to happen sometimes because mapping rules to a situation is subjective, more art than science. More importantly it might be a sign that the players are not visualizing the situation the same way you are (you think you're describing the water as storm-tossed, the players are hearing it as choppy but a nice night for a swim).
On the other hand if the same thing uses different rules on two different occasions, it's hard to see how it makes sense no matter who you are. This might just be the result of inconsistency (oops) or you might intentionally be using another rule to get an advantage.
The game world is imaginary. It does not exist except in the minds of the participants. Each person has their own mind and their own imagination, which makes it all the more important to make sure there is a consensus, that you are all operating in the _same_ fictitious world and in agreement about how things work. Consistency makes that easier, inconsistency makes it harder.
To use an example from M&M, the players encounter one machine gun that uses a normal attack roll, and then later they encounter another machine gun that uses an Area attack instead (automatic hit, Reflex save to reduce damage). Conceptually the two machine guns are identical — one is bigger but otherwise the same.
A player sees the second machine gun before it fires and says “a ha, I will dodge to increase my Defense, which will make me harder to hit!” Logical but completely incorrect, because that player doesn't know that the second machine gun uses a rule mechanic that has nothing to do with Defense.
Or to take another example, one supervillain has a lightning bolt that does normal damage and requires a Toughness save, but another character has a lightning bolt that requires a Fortitude save instead, bypassing force fields and anything else granting a Toughness bonus. Advantageous yes, but the person getting hit is unlikely to see it as consistent or fair.
There's a simple fix for this:
The same description should never be modeled with two different rules. If you want to use a different rule, there should be a different description.
In the lightning bolt case, emphasize that the Fortitude attack looks entirely different: it's a throbbing wave of static, not a lightning bolt. Does that seem like a trivial change? It's minor, but not trivial at all. A tiny change like can keep the game feeling consistent, which keeps the players involved.