Constantly changing ISO costs

Since January 2016 I have noticed that the costs to level up characters is constantly changing. The more ISO you acquire, the more it costs to level up characters.

For example, I have 15,325 ISO
-- A lvl 145 Ant Man (4*) which cost 92,043 ISO to lvl 250.
-- A lvl 145 Deadpool (4*) which cost 92,043 ISO to lvl 229.
-- A lvl 80 The Hood (3*) which cost 91,587 ISO to max out at lvl 166.
-- A lvl 30 Bullseye (2*) which cost 65,920 ISO to max out at lvl 94.

Now, spending this ISO below on lvl 120 Captain America (4*) which cost 79,153 ISO to lvl 188 actually changes the total ISO needed on other characters.
- 643, 644, 644, 679, 698 = 3,308
-- A lvl 120 Captain America (4*) which cost 75,845 ISO to lvl 188. --- diff 3,308 (spent)

ISO remaining 12,017
-- A lvl 145 Ant Man (4*) which cost 89,310 ISO to lvl 250. --- decreased
-- A lvl 145 Deadpool (4*) which cost 89,310 ISO to lvl 229. --- decreased
-- A lvl 80 The Hood (3*) which cost 89,136 ISO to max out at lvl 166. --- decreased
-- A lvl 30 Bullseye (2*) which cost 65,920 ISO to max out at lvl 94. --- no change

If I earn ISO, either by purchasing or winning battles, these costs for each character increases.

This is a problem for anyone if you click on the button to fully upgrade a character since you could be paying the inflated ISO cost. Like I said, I noticed this back in January when I purchased ISO for multiple characters needing around 74k ISO and after purchase the number was over 85k ISO required. It wasn't until this past week as I was trying to upgrade characters to champion where I noticed they were all increasing as I gained ISO from battles.

Please advise, Thanks,

Anthony