Synonym of "incorrect" with as little negative connotation as possible

I'm describing the results of an experiment I did, and I'd like to find a word to put in the blank (I'd also accept alternative ways of phrasing this statement generally):

We ran a computer simulation, using a very simplified model. The simulation predicted X. When we performed the experiment, we saw that the result was actually Y, showing that our simplified model was indeed _____.

In a different context, I'd normally go with "flawed" or "incorrect", but I'd prefer to cast this in as neutral a light as possible. After all, I didn't do anything wrong; I knew that using a simplified model would likely mean the results of the actual experiment would be somewhat different. If there is a single word that captures the idea "paints an incomplete picture", I'd probably like that.

Inaccurate would work well here; it doesn't imply total incorrectness, just that something didn't quite work out.

Try this:

We ran a computer simulation, using a very simplified model. Our hope was that the simplified model would lack precision but produce good results, and it predicted X. When we performed the experiment, we saw that the result was actually Y; our simplified model did not have enough fidelity to produce accurate results.

I think this matches the situation you have. You expected the model to be imprecise but accurate; what actually happened is that the model was flat-out inaccurate due to the lack of detailed modeling. (In my experience, the word to describe the level of detail of a model is "fidelity".)

This veers into English-editing territory, but I'd include a description in such a paragraph (or, expanded, section of a paper) about what you expected out of the model (which I stuck in above).

Mistaken has very little evaluative color, for me at least. Insufficient seems to fit too, at least here.

The best way to convey what you want to say is not to use a single word like flawed or incorrect. You can just use a positive word and negative construct, ie

We ran a computer simulation, using a very simplified model. The simulation predicted X. When we performed the experiment, we saw that the result was actually Y, showing that our simplified model was not precise/correct, etc.

This principle is used in teaching, when pointing out mistakes to students. In order not to discourage them, we don't use negative words, we simply say "This is not correct" instead of "This is wrong". I have found that this works effectively with people of all ages. I suggest you do the same here.