Sometimes matte tile can appear dirty even after scrubbing - I’ve been there, especially with older tiles. There are a few reasons why this may be the case.  The dirty appearance may be caused by stained grout since grout is porous and requires deep cleaning and resealing.  Another reason may be lower quality tiles or tiles that have been improperly cared for in the past and have been etched after the use of harsh cleaners.  The last reason may be residue build-up. For example, hard water can build up, causing stains.