You could have had enlightenment at first, which increases your XP gain multiplicatively by 4 times for a certain amount of xp gained. Once you use up your enlightenment, you will gain more 24 hours from the last time you gained it.
The other xp bonuses are all additive with each other.
With a 100% xp scroll + the 100% witches buff on a level 50 character (CP irrelevant), you'll get just under 34k xp from a consumable master writ (alchemy, enchanting, provisioning) and just over 50k from a gear master writ (blacksmithing, clothing, jewelrycrafting, woodworking).
Lower level characters earn less xp:
https://en.uesp.net/wiki/Online:Leveling
They also earn less gold, which would explain the 332 gold reward. A max level character would have earned a base of 377 gold for that consumable writ you turned in.
ETA: On the other hand, characters under level 50 don't benefit from enlightenment, so that doesn't explain a drop in writs turned in on that same character. Were you doing writs on characters of different levels?
It isn't possible to get 100k xp for a master writ even on a max level character with all possible xp buffs except if you have enlightenment. Knowing the exact number could help determine what's going on.