Sorry HaLeiVi, I know it may seem counterintuitive, but as I said this is a common misconception. I’ll try to explain.
Imagine an axis extending through the earth from north pole to south and extending into outer space. The sun from our perspective describes a circle around this axis once a day. The angle it makes relative to some predefined meridian is called the hour angle and it changes uniformly at the rate of 15 degrees per regular hour. The reason the length of the day changes is because in the summer more of this circle is above the horizon than below it, and vice-versa in the winter.
The simplest sundial has its gnomon (the stick whose shadow shows the time) pointing towards the north celestial pole (south in the southern hemisphere) i.e. parallel to this axis, and has its base 90 degrees from the gnomon i.e. parallel to the equator. The exact angles from horizontal and vertical will obviously depend on the latitude of the location.
On such a sundial equally spaced lines on the base 15 degrees apart extending round in a circle from the base of the gnomon will show equal regular hours of sixty minutes each. At the equinoxes the total extent of the shadow’s movement through the day will be 180 degrees, from the line marked “6 am” to the line marked “6 pm” (ignoring the equation of time (google it!) which sundials cannot take into account). In the summer the sun rises earlier and sets later, so the shadow will traverse more than 180 degrees, encompassing the lines before 6 am and after 6 pm, and in the winter the opposite, i.e. at sunrise the shadow will already be up to some line after 6 am and the sun will set before the shadow gets to six pm.
Actual sundials are more complicated than this and the marks are not equidistant. This is mainly due to the necessity of having a horizontal base, because in the winter the sun is always below the celestial equator so the sundial described above would have its shadow cast on the underside of the base (which was parallel to the equator remember). The principle however remains about the same. Making a sundial to show shaos zemanios is very complicated and involves changing the angle of the gnomon and/or the marks on the base throughout the year, and/or having a concave spherical-like base, all of which could not be done very accurately in ancient times.