The short answer
Glass thickness is not chosen by size alone. It is chosen by what the glass has to do: how far it spans, how high it sits above the floor, whether people walk into it, what is above and below it, and what it has to keep out. Two openings with identical measurements can legitimately need different glass because one is a bedroom window and the other is a shopfront people lean on.
That is why a price given over the phone is not a price. The number depends on a decision that cannot be made until the opening is seen.
Span and support decide more than area
A pane fails where it is unsupported, not where it is biggest. A tall narrow panel carrying wind load behaves differently from a wide low one, and a panel fixed on four sides behaves differently again from one held only at the top and bottom.
This is also why the frame, the head channel and the fixings are part of the glass decision rather than a separate trade. Glass held in a weak frame moves, and moving glass cracks at the fixings.
Human impact drives the safety requirement
The safety glazing rules exist because ordinary glass in the wrong place behaves like a blade. Glass in a door, alongside a door, or low enough that a person could fall into it is treated differently from glass above head height in a fixed panel.
In practice this is where most surprise comes from: a customer replaces a broken low window with the same glass as before, and the same glass is no longer what the location calls for. Replacing like-for-like in a location that was already wrong simply resets the clock on the same problem.
Make-up, not just millimetres
Toughened glass is heat treated so that it fails into small blunt pieces rather than shards. Laminated glass holds together because a plastic interlayer keeps the fragments attached to a sheet. Float or annealed glass is the ordinary form and is not suitable where the safety rules apply.
Adding thickness is one way to satisfy a span; changing the make-up is how you satisfy the safety requirement. They are different questions and both get answered before the glass is ordered — once toughened glass is cut, it cannot be cut again.
What we look at on site
The opening as built, not as drawn — frames are rarely square and floors are rarely level. How the glass is supported and what it is fixed into. The path people take past it: door swing, stairs, anywhere someone could stumble. And whether the glass is doing a job beyond looking right: thermal, acoustic, privacy, security.
The answer is written down in the quote — the glass, the make-up and the standard it is selected under — because a verbal agreement about "10 mil" does not survive a change of installer.