Reducing sauce or gravy always forms a skin
Not an expert at this, but I suspect the skin that forms on top is just the upper layers that gets slightly drier from being exposed to open air. By not being submerged like the rest of the sauce, fluids on top evaporate at a different ratio causing a more solid "skin" to form on top.

It can also happen on casseroles or sauce heavy dishes that go in the oven, and is probably an important part of gratin technique.

I don't think there is anything wrong with it other than potentially being unsightly. If you find it inaesthetic or unpleasant you can remove it at the expense of wasting a portion of the sauce.

You can however mix it back in without issues, or prevent its formation by stirring gently or from time to time so that the top layer never forms.

Math experts here can probably give you a more informed answer, but setting a ratio of evaporation per unit of time is hard because it depends on a lot of different factors, like the type of sauce, chemical composition, the total volume being cooked, area of exposure to air (wider or narrower recipient, lid/open air etc.), temperature/flame intensity, among others.