Sivir Spell Shield vs Veigar Baby Cage Inconsistency

While playing vs I noticed if i activate my spell shield and walk into a Veigar cage from the outside, it pops my shield and stuns me. If I do the same thing from the inside of the cage, I pop my spell shield and continue to walk onward.

I noticed this while playing ARAM and have admittedly done no further exploration into the subject, but I can't believe this is a 1-off thing given how it happened repeatedly during the course of the game.

My suspicion is that the knockdown ability of the cage is slightly larger than the stun ability of the cage, so when I walk in from the outside my spell shield blocks the knockdown and I'm then stunned, but when i walk from the inside the shield blocks the stun and the knockdown affects me with no measurable effect.