Concise answer
Strong overhead light, wet or oily hair and a wide part can make the scalp look more visible. A consistent increase in visibility may also reflect reduced hair calibre or density.
Phone cameras and harsh lighting exaggerate contrast, which is why daily mirror checks can be misleading.
Comparable photographs taken with the same hairstyle, light and angle are more helpful for detecting gradual change.
When evaluation may help
Assessment may help if the part keeps widening, the ponytail feels thinner, the crown shows more clearly or there is persistent shedding.