On an image page, we have $wgImageLimits that generate:

Other resolutions: 320 × 240 pixels | 640 × 480 pixels | 800 × 600 pixels | 1,280 × 960 pixels | 2,560 × 1,920 pixels | 4,032 × 3,024 pixels.

For WMF wikis these currently return different resolutions, namely: 340px, 960px, 1280px and 2560px
This is because of T360589 which added wgThumbnailStepsRatio

We now have differing wgThumbnailBuckets, wgImageLimits, wgThumbnailStepsRatio and wgMediaViewerThumbnailBucketSizes, all with differing values which is of course confusing, but image limits in particular being presented in the UI seems problematic.