Why does Fintel ownership data differ so significantly from other sites?

tldr: Fintel sources its ownership data from a larger variety of filings and therefore it is more complete.

In regards to the institutional ownership, most sites that report on institutional ownership use the traditional 13F filings as their source of ownership data. These filings have been around since about 2013 and are well-understood and well-known in the industry. About three years ago, the SEC introduced a new filing - the NPORT filing, which all US-domiciled mutual fund and ETFS are required to file. We of course began processing these and reporting on these immediately but surprisingly, many other sites have not.

Additionally NPORT filings require funds to report on short positions, bonds, and other non-equity filings, which is why you see that data reported on Fintel and not on other sites.