I can only spot check the two that I've purchased, but it looks like the math is wrong.
River Kingdom Excavation says 'May purchase more than once, but cost increases 50% at each purchase' It starts at 4 PTP, but shows 12 PTP for a second purchase. That's not a 50% increase. 50% Increase would suggest 6PP for the second one.
Experienced Adventurer says 'May purchase more than once, May purchase more than once, but price doubles at each purchase.' starts at 12, lists 48 for a second purchase. Double would suggest 24.
In both cases it looks like an extra factor of two is either being added to the cost, or is missing from the test.