Phoenix:
You were around back in the early days of the BOD system, so perhaps you can answer -
I can't speak to the original intent, as it was another designer who built the system. It wasn't until adding the bribery stuff that I spent much time on it other than just doing BODs in-game.
Here's a complete rundown of how it generates deeds.
BLACKSMITH:
1. If your skill is under 70, you only get small non-exceptional iron deeds for items you can craft.
2. When skill is 70 or more but below 100, chance of exceptional is 33%
3. At 100 skill and above, chance of exceptional is 50%
4. When at 70 skill or above, large deeds happen 8% of the time
5. Large deed breakout: 25% Ring, 25% Chain, 25% Plate, 5% Axes, 5% Swords, 5% Fencing, 5% Maces, 5% Polearms
6. For all armor deeds, large and small, the chance of it requiring colored metal is: 60% at 100 skill, 70% at 110 skill, and 90% at 120 skill.
7. Skill affects chances of colors and limits available colors. For example, At 70 skill, you can only get Bronze and below; at 80 skill you can get up to Golden. Skill above 100.0 does not improve the chances of the rarer colors.
TAILORING:
1. If your skill is under 70, you only get small non-exceptional deeds for items you can craft.
2. At 70 or above but below 100, chance of exceptional is 10%. At 100 Tailoring, chance of exceptional is 33%.
3. At 70 and above, 8% of deeds are large
4. Each large type has an equal chance of being generated
5. Below GM, chance of leather deeds requiring colored leather is 50%. At GM, the chance improves to 70%.
6. Skill affects leather colors generated; for example, you can only get Horned deeds above 80 skill.
If I were to speculate as to why Tailoring's chances for exceptional and colored deeds are lower than for blacksmiths, I would guess that it has to do with how tailoring has such a smaller variety of deeds (only 3 special materials instead of 8). Even with Tailoring's lower chances of exceptional and colored deeds, it's still far easier to assemble a top-end large Tailor BOD than it is to assemble a top-end large Blacksmith BOD.