Because a percent change is based on a comparison of the change in value the starting value and if the starting values are different, then the percent change is different.
When you increase from 45 to 75, you're comparing the change in value (30) to where you started (45).
When you decrease from 75 to 45, you're comparing the same amount of change (30) to a different starting value (75).
30/45 ≠ 30/75
