Part of the confusion with the non-matching items from Sunwell is that at least some of them (Warrior ones certainly did, but druid ones didn't; I can't remember about my paladin) used to have the matching graphic to the Tier 6 set until around Cataclysm, when they changed all of them to match the Sunwell set graphics instead of Tier 6 for transmog, as the Sunwell sets lacked belts/boots in many cases.
I guess some people don't realise that the graphic was changed, and remember them matching before
There's also the fact that I guess people who didn't play back then don't realise the history of those 8-piece sets and assume same set-bonus = same set when they wowhead it. For anyone interested in a little history - when they released 2.4 with Sunwell raid, there was no official tier set as it was a bit of an unplanned bonus raid (tier 6 was in BT, tier 7 wasn't until WotLK). They put in belt, bracers and boots from the early bosses in Sunwell and made them attached to the old tier 6 sets so that you would be able to switch your chest/legs/helms from BT tier 6 loot to the new higher ilvl non-tier Sunwell loot without sacrificing your powerful set bonuses in the process. It was a useful idea.
But yeah, nowadays the tier 6 set's matching boots/belt for
every class drop as non-set items in BT/Hyjal, while the actual items with the set bonus (Sunwell drops) match the Sunwell transmog sets.