Redfin is an actual real estate brokerage, and are therefore real members of the MLS, with access to the data that provides. The others are simply scraping via publicly available data. This is a major difference.
Is it a major difference, or just a matter of timeliness? The actual data that Zillow etc. has seems identical to any other service, and the same as the printouts that realtors give out that they are printing from MLS.