With lyrics and prose, writers can take liberties with how to pronounce words. Although those characters are usually read as "bokutachi," Ayu has decided to replace the reading with "bokura" (which means the same thing). She does the same thing in other songs (evolution).

Another example of Ayu replacing readings is in 'evolution.' In the line with the characters for "chikyuu" she replaces the reading with "hoshi." Although the meanings are slightly different, a planet is a star so it fits in context.

This happens a lot in novels and the changes in reading are usually marked with furigana.