Becky,
We've encountered this a lot.. whether the challenge wording is strictly enforced varies a lot. And often kids will see previously-approved challenges and conclude that as long as their challenge is similar that it, too, will be approved.
Of course it's understandable when you have different mods approving challenges that there will be differences in assessments.
But kids often have an exaggerated sense of fairness/justice and something seemingly minor is a crisis of injustice. On the other hand as a parent, I appreciate that if a challenge has stated requirements, that the mods only approve challenges that meet those requirements. I'd like my kids to be encouraged to be mindful of what they're doing.
And then you have other factors. For example let's say the challenge is "Paint a Still Life" - and you have one kid who painstakingly creates an awesomely detailed oil pastel still life (which is not technically a painting but for which there is no other challenge) yet another kid slaps something together a clearly hastily-done watercolor painting of an apple. The second gets credit but the first one does not. And then it adds insult to injury when you go back and see previous still life *drawings* have been approved for the challenge.
I'm not sure what the solution is, but for consistency's sake, you have two basic approaches to choose from, each having pros and cons:
1) Allow for very liberal interpretations of the challenges and approve anything that has precedent for being approved.
2) Make a policy for strictly adhering to challenge requirements and somehow archive old ones that don't meet the stated requirements.