@Porkisgood it's a 24 hour measure. If a player drops then rises back to his original price it will show an increase.
A £1 player drops to 90p at 11am. This shows as a 10p decrease.
At 2pm he goes back to £1. This will now show as level.
At 11AM the next day his price will show as 10p up as it is comparing to 11AM the day before when it was 90p
At 2pm it will go back to level.
It's easy to get caught out if you haven't checked in a while and see the big increase. Watch pogba today. If he holds at his current price he will go to the top of the increase list even though his price is the same as two days ago due to the 60p drop and 60p gain yesterday.