NCEs are another transformation of z scores (standard deviations), just done in such a way that (in a normally-distributed population) a NCE value of 1 = percentile 1, NCE 50 = 50th %ile, and NCE 99 = 99th %ile. Each NCE unit is a fraction of a standard deviation.

Percentiles are an ordinal measure, which describe your ranking in the norm group.

If you have a non-normal distribution of scores, the NCE and percentile will line up even less well.

...pronounced like the long vowel and first letter of the alphabet...