I don't think that's necessarily the case. I've seen posts saying that they count as +X towards the +10 limit, with the only difference being a flat cost.
Page 462 Core Rule Book
"Magic Armor and Shield Special Ability
Most magic armor and shields only have enhancement
bonuses. Such items can also have one or more of the special
abilities detailed below. Armor or a shield with a special ability
must have at least a +1 enhancement bonus."
It's for Game Master use when rolling loot. Glamored on armor is a good example, flat cost of +4000(2,700 with Ultimate Equipment) gold in the core rule book page 463. Here is a chart where the game master would roll and find out random armor enchantments and flat bonuses to the armor(Or weapons). This has no effect on the +10 hard cap of non-mythic items(other than making it sell for more!).