If you notice that a certain score in a game earned you, say, 800 NP one day and only 700 NP the next, do not worry, this is not a glitch.
In an attempt to balance the number of Neopoints earned among all the different games Neopets has to offer, the NP to score ratio for any particular game may change from time to time, so be sure to keep an eye on the ratio for your favourite games!