Sethlon and I figured it out, and its really really gay lol. I'll explain it more when I have more time.
EDIT: So this is what happens. Keep in mind all of this is with projectiles and wasn't tested for physical hurtboxes.
Everyone has a sphere-like shield except for Yoshi. For everyone else, their hurtboxes barely move when they put up their shield and it covers almost all, if not all of their body. When any hit box comes in contact with a character's shield during the first 6 frames, it counts as a powershield. However, you can shield stab if a hitbox hits a hurtbox without touching the shield.
Yoshi is different. When Yoshi gets in his shield (which is egg-shaped), his hurtboxes move considerably since he has to bend down and curl up to get in his shield. This would not be a problem if Yoshi's shield was sphere-like. However, because it is egg-shaped, it does not cover his nose properly, and projectiles can end up hitting his nose as he bends down within the first 6 frames without touching the egg-like PS area. Thus, it pokes Yoshi and hits him.
To give you a good idea, I'll give an example I did. Falco and Yoshi are on FD. Falco shoots a laser at Yoshi, and it hits Yoshi. You put Yoshi in the same exact spot and 1 frame before it hits, you shield. If the laser is close enough to Yoshi, it will touch the PS area and you will powershield the laser. In the same exact spot, you shoot another laser, and this time you shield 2 frames before the laser is supposed to hit your hurtboxes. Yoshi bends down on the next frame and extends his hurtbox, getting hit by the laser, 1 frame before you were supposed to get hit.
Basically, the hitbox has to be close enough to you in order to PS it. If it isn't, it will end up hitting your nose. You can PS on frames 1-3. Starting on frame 4, it is a normal shield. Because the nose is the only issue, if your back is facing the projectile, you will PS on frames 1-3 every time with no worry.
In essence, you will never really know if the attack will powershield if it is anywhere near the vicinity of your nose. If you plan to powershield a projectile, put your back to the projectile and it's much easier and reliable.
This whole argument also extends to physical attacks. We thought that physical attacks were somehow different than projectiles for shielding with Yoshi, but it's the exact same. The only reason we don't have the same problem is because almost all physical attacks have much larger hitboxes and end up never shieldpoking because they extend to our PS area. It doesn't matter whether the hitbox touches the hurtbox as long as it also touches the PS area as well. It counts as a PS then. It's only when it ONLY touches the hurtbox.
Like I said...gay lol
I have my Action Replay disc around here somewhere...
