Perfect clean weapons...

Am I right in assuming that there isn't an equal chance for every possible attack stat a weapon could have?

As in, a 5 above average clean weapon is far rarer than an average one, not just in higher demand?

Because I've been testing a few things, and long story short, I went and made a good 30 concertos each with an advanced diamond + stimulator and the best thing I got was a single 80 attack clean. With almost all of them being 77, which means out of 30 tries the best I got was 3 above average.

Or am I unlucky/ missing something?

I don't know the numbers, but the chances are probably in percentages. Because I've done the same as you with weapons, however I've at least got one to go 5 above average once. All about luck.

maker skill can give you up to 10 stats above average. Highest i achieve was 9 above.

Yes, higher random stats are much rarer - I've found maybe 2-3 perfect weapons in the time I played MS.

You can only go +8 above average with a diamond and stimulator unless you're using black crystals as well.

Yes, higher random stats are much rarer - I've found maybe 2-3 perfect weapons in the time I played MS.

You can only go +8 above average with a diamond and stimulator unless you're using black crystals as well.

And with an Advanced Black Crystal, that'd add an additional +3, correct? So a perfectly-perfect weapon would be +11 ATT higher than its clean counterpart?

I've seen a few perfect weapons. I came pretty close to making a perfect Red Craven a couple months ago, but instead of using an Advanced Black Crystal I used an Intermediate one and got +10 attack higher than average. I do believe there was a perfect one in some other world, but I've yet to see it.