Am I imagining it?
Your horse is compared with himself, not with an average horse. A change becomes a figure you can point at.
Which leg is he sparing?
Every leg gets its own deviation percentage for each gait, so the uneven side shows up on its own.
Is it him, or is it me?
Your vertical movement is measured against his back, in centimetres and milliseconds. If the imbalance follows you, you will see that too.