That's because each skill line main passive has different ranks, although all go from 1-50. For example:
- Jewelry Crafting: Engraver has 5 ranks
- Provisioning: Recipe Improvement has 6 ranks
- Alchemy: Solvent Proficiency has 8 ranks
- Enchanting: Potency Improvement, Blacksmith: Metalworking, Clothier: Tailoring, Woodworking have 10 ranks
So doing writs with a character that has all those skills leveled to 50 and the main passive fully allocated will give containers with different ranks, although all of them are maximum rank and have a chance of providing master writs.

Provisioning is actually an odd one since it will ask for CP50 food/drink half of the time and CP150 food/drink the other half, so you will receive Rank V and Rank VI respectively when you turn them in; note that only the latter will have a chance of containing a master writs. That's the main reason why provisioning master writs are relatively rarer compared to other consumable ones.