This effect is commonly called green non-uniformity (GNU). A typical cause is pixel crosstalk: the two green sites in a Bayer pattern sit next to different colored neighbors, so differences in how signal leaks or spreads from adjacent red/blue pixels can make the two greens respond slightly differently even on a uniform area.

In practice, this is treated as a systematic sensor non-uniformity rather than random thermal noise. It can also overlap with general pixel response variation, but the key point is that the mismatch between the two green channels is often spatially structured and repeatable.

A common mitigation is to treat the two green channels separately, applying different green gain factors and correction algorithms during image processing.