What does point-cost difference mean?

The wording of this rule is just confusing me.

To improve one of your basic attributes, you must spend character points equal to the point-cost difference between the old score and the new one.

What, exactly, does "point-cost difference" mean? Let’s assume I have a ST of 13, and I’m attempting to advance it to 14. Mathematically, each level only costs 10 points, so the difference in the old cost vs the new one is literally zero (subtracting 10 from 10). Common sense tells me that can’t be right…

Do they mean the cumulative point cost difference? Well, in that case, at character creation, a ST of 13 would cost me 30 points, and ST of 14 would cost 40. 40-30 = 10. Every new additional pump into the attribute will cost me, cumulatively, 10 more than the previous one at character creation, so the "difference" between the costs is 10, always… And common sense tells me that surely this is also wrong.

Using context from other game systems, I’d guess that in my example I’m supposed to spend 40 points (30 cumulative for 13, 40 cumulative for 14, so by that logic, when "leveling up", I should spend a full 40 points to get from 13 to 14). If that’s indeed correct, then I could really use your help finding proof to back it up. Does any official documentation exist (something akin to D&D’s sage advice) that corroborates this?

To note, I’m referring strictly to level advancement, not character creation.