it's nymphs in general...
I saw a post ages ago that states the electrum amount is randomised per card, e.g. Dusk Shroud will sometimes give you 15 on a spin, but other times it will give you 25 (i don't know exactly)...
Nymphs give you either 300,600,900 or even 1200! so your nymph there, althought it is soo much better than normal, was the lowest it could have been >_<
my white nymph got me 600