Different ISO amounts to level different 5-stars

sambrookjm

I've got a question about the ISO required to level up the 5-stars.  I currently have three five stars with six covers, all of whom are at LVL 255:

Spider Man (Peter Parker) is at 2/1/3, and would take 212,504 ISO to reach level 345.
Hawkeye is 1/3/2, and would take 210,740 to reach level 345.
Ghost Rider is 2/3/1, and would also take 210,740 to reach level 345.

My son has multiple one-cover 5-stars, and Black Bolt requires a different amount of ISO to reach his max level than the rest:

Black Bolt (0/1/0) - 24,162
Archangel (0/1/0) - 22,476
First Avenger Cap (1/0/0) - 22,476
Ghost Rider (0/1/0) - 22,476
Iron Man (0/0/1) - 22,476

Has anyone else noticed this, or been given an explanation as to why this is happening?  Does it happen with characters that are 4-stars and below?