What is the proper way to measure the cut-out for the kitchen range?
Typically a "drop-in" range will rest on the adjoining counter top surfaces.
That the actual cabinet is narrower, gives you the wiggle room you need to drop in the unit to install it.

If you want to change to a cooktop installation, that is something that is only supported by the counter top, then you'll need to build a cabinet, and provide a counter top with cutout to fill that space.

In any case, as others have suggested, check the exact measurements of any product that you select.  In this age of the internet, most manufacturers have their Installation Guides online, you can pull a copy in spite of the retail staff.