When players buy fuel, they’re converting real money into a game currency which makes the transaction feel less like spending money.
Using marketing prices like 499 instead of 500 when spending that currency only reminds players of real-world pricing tricks.
It reduces revenues both directly because of the unnecessarily lower price and indirectly because it makes the experience feel cheaper and less enjoyable.
This kind of pricing makes sense when buying fuel with actual money (like $99.99 instead of $100), but not when spending it in-game.
It's one of the rare cases where I would enjoy spending more... take my money please!
PS: this reasoning can be applied to any in-game purchases, not only d1sks.