Water flows at a rate of X. If the water is on top of ice, items (or xp orbs) will travel quite faster than X.

One would expect the item to travel no faster than the water, but it ends up going several times faster. (The two screenshots are meant to show this, though if it's possible for someone to submit a .gif or something of the sort that would be much appreciated)

To reproduce:
1. Lay down several ice blocks in a row
2. Surround the air blocks on top with any water-stopping block
3. Place water at one end of the ice
4. Drop item on the water source

This setup should show that the item flows faster than it should.