target bonus at meta

How does the target bonus base percentage work at Meta? For example, I see a lot of TC offers on here that include the bonus amount (not sign-on), however, it appears that the base percentage amount is determined based on your level and is not something that's negotiated during the hiring process. Is the reason it's displayed on this site because the current UI is just a catch-all that accounts for those companies where it is negotiated? If it is, it's still weird because some offers I see — say for E4 — are not always 15%; most of them are, but saw another for base 180K and bonus 20K. Passed the on-sites, currently in the hiring committee phase for E4.