It's better to go by the percentiles to tell you more specifically where you stand. If you have a 90 percentile, that means you scored in the top 10% of the test takers that they compare you to. A 75 percentile would mean you scored in the top 25%, which is better than a 75% on a school test, which would be a C.
Your scores are fine for most private schools. For the most rigorous, they might be a on the low side with the 7 and 6, but those schools are very few, maybe 1 - 3 in the biggest cities, and even then, the lower scores might not be an issue. If you attend a very rigorous private K - 8 and get all As, but just aren't a good test taker, they understand about the lower scores. They'll see your past grades and standardized test scores. It's better to get high grades consistently rather than great ISEE scores, because that shows you're willing to do the work. Anyone can score high on one test.
The ISEE compares to the top students, private school students, so it's tougher than a typical standardized test, the standards are higher. So you did really well!
Good luck with admissions!