HQ Items question

My question is people says the HQ item already come cap in the stat they show. For example Vanya of healing come with mnd +20 and spell speed +19. they say I cant attach any more mnd materia because its already cap by the HQ... but I see you can add 2 spell speed materia of +6??? can anyone explain me what is the issue with this? is there is a guide or a webside covering all of this? Thank you,

Items have stat caps. You can attach spell speed materia because the cap for spell speed is significantly higher than it is for the primary stats (STR, DEX, VIT, INT, MND, PIE).