Don't forget, it's only a scale of 1 to 6. This means that, at the very least, that every person UP TO Luffy's level withh be placed on a number between one and six.
So if a random marine is a 1… and Luffy's a 6... there's only four levels of strength in between?
If Sanji is a five, would Arlong place a 5 too? He is obviously weaker than Luffy, so they're not equal. What about Blueno? Would Bleuno be the same as Sanji and Arlong at 5? A 4? What about Jabura? He lost to Sanji, but would that make him a 4? But it's obvious Jabura is much stronger than Blueno... so is Blueno a 3? A 2, even? Just above a random marine? Where would Django fall? Blueno's level, a step below?
You see what I'm getting at? There simply isn't a large enough range of numbers to accurately place everybody's strength. The best we can say; "On a scale of one to six, they're equal." But people on the same level at one through six don't have to be equal in fighting strength. If both Blueno and Jabura place a 5, they don't have to be equal. If Jabura is the same as Sanji, it doesn't make him and Sanji equal.
Now I can hear the counter-point now, and let me head it off. "But Oda planned it through. If he needed to show the difference between Zoro and Luffy, he would have used a larger scale going beyond six."
And here's the counter-point to that. I agree Oda wouldn't do somehting like that without thinking about it; but what if it was his goal to remain vague about their relative power? What if he didn't want to reveal who was the stronger, leaving it to inferences in the manga to show us?
That doesn't make them equal, it means Oda left it as vague as "They're on the same playing field."
Which seems very much like Oda to me.