They are very flakey. I think its because they draw in data from as many sources as possible, and if there is a problem with any of those sources it throws everything off. For example the nxt price was because one exchange with low volume was quoting a very, very high price - probably somebody did a market buy and some tiny amount, like that last 1 or 2 nxt to complete the order, was picked up way off market price, but because that was the last price someone bought at its what gets reported to CMC.