"Oily", "dry", "combination", "normal" — the four boxes on the back of every questionnaire. They are useful shorthand, and they are also the source of a lot of unhelpful decisions.
What the label is describing
Skin type mostly describes how much oil your skin tends to produce. That is genuinely useful information: it predicts which textures will feel comfortable and which will feel heavy or insufficient.
What it does not describe is hydration, sensitivity, barrier condition, or how your skin will behave under a particular ingredient. Those are separate qualities, and they move independently.
Why it changes
Oil production is not fixed. It shifts with the season, with indoor heating and air conditioning, with hormonal cycles, with age, and — often most dramatically — with what you have been putting on your face.
Skin that has been stripped by strong cleansers frequently produces more oil, which reads as "oily skin" and invites more stripping. Skin in a dry winter flat can read as "dry" when the person's underlying tendency is not dry at all.
Combination is the normal case
Most faces are not uniform. An oilier centre panel with drier cheeks is so common that it is closer to a default than a category. It is entirely reasonable to use a lighter texture in one area and a richer one in another.
A more useful question
Instead of "what type am I", it is worth asking what your skin does across a normal day. Does it feel tight by mid-afternoon? Shiny by lunchtime and tight underneath? Comfortable until something specific sets it off?
Those answers point at what to change. A one-word type rarely does.
