So there are two possibilities:
It's noon, and we have an off-peak discount subscription of some sort. Check in at Amsterdam Central, travel to Maastricht, check-out at Maastricht. Fare price: €15.
It's still noon, we still have the same subscription and yet again we check in at Amsterdam Central and travel to Maastricht, but clumsy as we are, forgot to check out. No worries though, as this results in the fare being only €10! So we're cheaper off than when we would have checked out.
Now we're in fact not clumsy at all, we're actually Hollanders, calculating well and doing this trick every week (twice, once on the way to Maastricht and once on the way back), saving us €520 per year. Not bad! All we have to do is in Maastricht, check in with Veolia and immediately check out again (no charge!) to prevent a check-in at Maastricht with NS to be processed as a check-out.
The question isn't whether or not this will work. It works. The question is rather how long it'll work and why fares above the check-in deposit actually exist in the first place.