The Steamers and Drifters here are calculated using the current back price and the WAP (Weighted Average Price). This gives us a more accurate representation of the 'movement' of the prices.
Comparing the early price to the current price is not as accurate as the early price will not have actually been traded much, if at all. The WAP price is the actual average price the horse has traded at. If the price of a horse moves, its average will move a small amount (depending on the amount traded) but the actual price will move more thus creating a difference between the two. If you want to see this in action take a look at the Bet Mover Markets app, click on a horse name and you see the fluctuations between the back and WAP prices, here's an example:
We can clearly see that at the point indicated the back price was a clear drifter from the WAP price. 12.5 to 17.5 = 40%. Its early price was 10 giving a movement of 75% - this is too high and is not a good representation of the actual amount of change. As we can see the price falls back to 13.5 (not quite 12.5 but certainly no way near is early price of 10).
Steamer - A horse whose odds has shortened considerably
If a horse is backed relatively more than other horses in the race and more than its layed 'backed to lose', it will reduce in odds.
Drifter - A horse whose odds has lengthened considerably
The converse is true for horses that are layed (backed to lose) or if other horses are backed-in and as the book needs to balance the price rises as a consequence.
It's a supply and demand thing.