I vaguely remember reading, no reference comes to mind, that the first Glengarries were based on a hat, similar to the Balmoral, that someone blocked by pulling on the center front and back. This then lead to hats being knit to a Glengarry shape rather than a round hat pulled front and back. As a knit hat there would be more natural bend/curve for a knit hat, even a knit and felted hat would have the bend/curve.

The current manufacturing techniques for the Glenngarry doesn't have the give for the bend.

FYI "Folk Hats 32 Knitting Patterns & Tales from around the World" by Vicki Square, published by Interweave Press has a pattern for knitting a Glenngarry.