ICO -> PNG not always same quality

[tbee]

When I convert a 128x128x24b ico file to png I expect something that is equally good, like a 128x128x32b PNG, but sometimes a 72x72x24b file is converted to 72x72x4b (24b->4b) And that looks awful! How come using the same settings I get different output?

[xnview]

When I convert a 128x128x24b ico file to png I expect something that is equally good, like a 128x128x32b PNG, but sometimes a 72x72x24b file is converted to 72x72x4b (24b->4b) And that looks awful! How come using the same settings I get different output?

Perhaps the 4bits image is the first in .ico...