They honestly don't work like that. It might appear to be giving him +15 str, but really only giving him 3. I have Soulfrost on my warlock that is doing that exact thing, showing their full values but only giving the scaling values.
imgur: the simple image sharer In my bags, shows the actual value. On my toon, shows the value before scaling.
That's why it's important to actually test that value against what you see on your character sheet.
I can't tell you exactly how Blizzard implemented the scaling or what they did to address enchants on people's gear before the patch, but I can guarantee you that there are plenty of enchants that are showing bugged tool tips and some which aren't even down scaling properly. Elemental Force is proccing for 110-220 damage for all classes at all levels below 60, instead of having a specific range dependent on which level you are. Landslide is doing the same thing, proccing for 75 AP regardless of your level below 60. Other enchants, such as +15 agi to glove show one value and give another. On my 29 it shows as +8 agi, but gives only +3. On my 39 it shows +10 agi, but gives only 3.
Scorched Leg Armor says it gives +10ap +12 crit @ 29. It actually gives +4 ap +4 crit. Peerless Stats (and other +stats enchants) actually only give +1 to stats, regardless of what the tool tip actually says. Exceptional Health to chest is probably BiS @ 29/39, as it gives +60 hp/+77 hp. 1 stam is only 23 HP @ 29, so +60 hp is definitely better than Peerless Stats.
These are just examples and precisely why you have to actually test the enchant vs the character sheet numbers before you assume it's just GF'd. Enchant values are the most unlikely thing to be GF'd, even in Cata when 5.4 came out and some people had unbound Parachute Cloaks with +12 agi on them. Just because they display one value, doesn't mean you can actually trust it to give that value.