Customers don't arrive at predictable, evenly-spaced times.
Example: Everyone wants lunch at noon
Customers ask for different things, some complex.
Example: Custom orders vs. standard menu
Customers have different abilities to perform their role.
Example: Tech-savvy vs. tech-challenged
Customers put in different amounts of effort.
Example: Reading instructions vs. asking staff
Customers have different opinions on what "good" means.
Example: "Fast" vs. "thorough" service