Why do shower quotes vary between companies?

Depending on the type of system, whether it is a sliding door, Swinging or Pivot Hinges, or maybe Bi-Folding, the price may vary.
Sometimes a project may at first glance seem similar to another, but may differ, for example, in the color or thickness of the glass, which can be 3/8 inch, or maybe 1/2 inch.
Not unimportant factor affecting the cost is the type of fastening of glass panels to the walls. Metal clips can be used, which means additional cutouts in the glass and the cost of the clips themselves is much higher than, for example, a U-channel that fits around the entire edge of the glass panels and gives a framed look to a frameless glass shower.
The choice of style for clips, rails, doorknobs, towel rails and robe hooks can also affect the price.