A good half *if not more* of the AGP slots out there don't have full AGP complience either.
Did you forget Intel's 875 chipset class (e.g. MSI 875P Neo)?
Hell, I've yet to run across a single AGP 8x vendor that has full complience.
One can only say this statement IF one doesn’t keep track of X86 world (specifically Intel Corp).
(don't forget, AGP 8x complience means AGP 2x or slower cards no longer work, so if the slot can run an AGP 2x card, then it's not fully 8x complient)
Depending on the standard e.g. “Type 4” vs “Type 5” vs "Type 6".
Referencing Intel's AGP 3.0 Interface Specification Rev 1.0. Page 39.
There are 6 types of AGP equipped motherboards;
1. AGP 3.3 V Motherboard, 3.3 V keyed, Supports only AGP 3.3 V signaling. Available speeds 1x, 2x.
2. AGP 1.5 V Motherboard, 1.5 V keyed, Supports only AGP 1.5 V signaling. Available speeds 1x, 2x,
4x.
3. Universal AGP Motherboard (UAGP)
Universal (UAGP) Supports both AGP 1.5 V and 3.3 V signaling. Available speeds 1x, 2x, 4x.
4. AGP3.0 Motherboard, 1.5 V keyed, Supports only AGP3.0 signaling. Additional electrical ID to
prevent AGP 1.5 V operation. Available speeds 8x, 4x.
5. Universal 1.5V AGP3.0 Motherboard (Universal 1.5V AGP3.0), 1.5 V keyed, Supports AGP 1.5 V and AGP3.0 signaling. Available speeds 1x, 2x, 4x, in AGP2.0 mode and 8x, 4x in AGP3.0 mode.
6. Universal AGP3.0 Motherboard (Universal AGP3.0)
Universal (UAGP) Supports AGP 3.3V, 1.5V and AGP3.0 signaling. Available
speeds 1x, 2x, 4x in AGP2.0 mode and 8x, 4x in AGP3.0
mode. This includes 3.3V support for 1x and 2x speeds.
Manufactures can select different solutions for their needs.
(don't forget, AGP 8x complience means AGP 2x or slower cards no longer work, so if the slot can run an AGP 2x card, then it's not fully 8x complient)
You are only referring the Type 4 AGP motherboard.
"the terms AGP1X, AGP2X, AGP4X, and AGP8X refer to the speed of data transfers on the AGP Interface while AGP1.0, AGP2.0, and AGP3.0 refer to specific releases of the AGP Interface specifications." - page 11
Examples;
ASUS nForce2 (A7N8X) supports "1.5V AGP 8X Pro" e.g. "Universal 1.5V AGP3.0 Motherboard" (Type 5).
(nVidia nForce 2 Class).
MSI K7T Turbo-R (MS-6330 V3) supports AGP 2.0 e.g. AGP 1.5 V Motherboard (Type 2). (VIA KT133A Class)
MSI 875P Neo (MS-6758) supports 8x/4x at 0.8V (AGP 3.0) or 4x at 1.5V (3.3V is not supported) e.g. AGP3.0 Motherboard (Type 4). (Intel 875 Class).