Why do face similarity scores change with different photos?

If you compare the same people using different photos, the percentage may move. That is expected because the model analyzes the image it receives, not an unchanging physical face in perfect conditions.

A photograph is only one view of a face

Faces are three-dimensional and constantly changing. A photograph freezes one angle, one expression and one lighting condition. Turning the head changes the apparent width of the nose, distance between features and visible jawline even though the person has not changed.

Lighting changes visible structure

Shadows can hide one side of the face or emphasize contours that are less visible in another image. Flat lighting may reveal features differently from strong side lighting. Backlighting can reduce detail and image contrast.

Camera distance and lens perspective matter

A close phone selfie can exaggerate central features because of perspective, while a portrait taken farther away may look flatter and more natural. Comparing images made with very different camera distances can therefore influence the numerical representation.

Expression changes geometry

Smiling raises the cheeks, changes the eye area and alters the mouth. Squinting, laughing or looking surprised can create additional differences. More neutral expressions often make repeated comparisons more stable.

Age and appearance change over time

Children's faces develop quickly, while adults may gain or lose weight, grow facial hair, change hairstyle or simply age. A family resemblance may still be obvious to a person while an image-based score changes because the visible details changed.

How to test consistency

If you are curious, use two or three clear photos of each person. Keep camera angle and lighting reasonably similar. If results remain close, the comparison is more stable. If they vary widely, the photos themselves are strongly influencing the estimate.