Quote from macal425:
Although on 'average' YM may move 10pts for every 1 ES point move, it is an average. Within that, you are going to get days where one shows relative strength over the other and vice versa. There can be different reasons for the divergence. For example, if you get just one or two dow components that are very strong on the day( due to earnings, news etc...), in comparison to the rest of the market, then there is a good chance that the dow(YM) will show relative strength over ES. It can also be something as simple as one of the indexes reaching a support/resistance level and taking a breather while cash rotates into another index.