Why does Proton Pass Plus cost more on the 24-month plan?

I was looking at Proton plans, and I noticed that the Proton Pass Plus plan costs almost double on the 24-month plan compared to the 12-month one. 1 month: USD$4.99 12 months: USD$1.99 24 months: USD$2.99 The Pass Plus plan is the only one that gets more expensive at 24 months. Why is that?