Part of the confusion is from there being two parts of failure, and failure being something rolled against multiple times...
If you had an item that was 10/10/10/10/10 and were enhancing it with something that changes all 5 resist properties, the chance of it failing is
A. 30% on Physical
B. If A does not fail, then Fire is checked (30% failure)
C. If both A & B do not fail, then Cold is checked (30%)
D. If A-C do not fail, then Poison is checked (30%)
E. If A-D do not fail, then Energy is checked.
So, the chance of a successful enhancement in this case requires 5 checks.
So this works out the chance of failure (at 100 skill) is
30% + (30%*70%) + (30%*70%*70%) + (30%*70%*70%*70%) + (30%*70%*70%*70%*70%)
or
30% + 21% + 14.7% + 10.29% + 7.203% = 82.993% chance of failure (both catastrophic and failure where the item survives, combined).
The chance of a catastrophic failure (CF) is a function of the failure chances. The chance of CF appears to be roughly 3/5 to 5/9 of the total failure chance. My guess is that the checks for catastrophic failure are made each part of the enhance step, so that there are a bunch of smaller individual checks for CF instead of one at the final step - and I suspect that even if the failure is at the first step, that all possible failure/CF checks are still made.
Checks are also only made for properties that change values, which makes Spined much easier to enhance with than other materials, as only checks for luck and physical resist are made, so long as luck is low to start (in fact, it takes about 75 luck on something with all-5s resists for a spined enhance to be as difficult as a horned or barbed enhance of the same item). Woods, most metals, and the other two leathers, add to 4 to 6 properties (only DC adds to as few as 3) - Each additional property that changes value introduces a significant reduction in success, regardless of how much the property changes.
I'm going to see about getting the full numbers added to the descriptions under the calculator as to how the failure chance gets broken into the "Breaks" and (unlisted on the Stratics calculator, but shown on ToR) "fails but survives" segments.