The following are two "Healey" explanations for the creased bonnet.
as explained by
Jim Werner of the Bluegrass Club.
The Creased Bonnet

 100-6 Models sometimes came with a creased bonnet. One of the mysteries of Healey production, they appeared, disappeared and reappeared in no logical order. Here are two explanations from Donald Healey and Geoff Healey.

. When I saw Donald M. Healey on one of his visits to the States, I asked him specifically about the creased bonnet, as I have on my 100/Six. He claimed that when they ran low on the creased ones, they reordered and they came through without the crease, so they continued to install them on the cars. A while later they found a crate of creased ones in the warehouse and installed them. As he said, "Don't you understand, we did anything to sell cars to you Yanks." 

Rudy Streng

This is a great example of how much or how little we know about our cars since the original documentation was sparse and mostly destroyed after the British Leyland merger. I asked
Geoff Healey about the creased bonnet (and remember that Donald did the visionary part and the public relations work, while it was Geoff who was responsible for going to Longbridge and then Abingdon to approve changes on behalf of DHMW ltd.).

His explanation was that the crease was included when the
first six-cylinder bonnets were designed because they were only braced at the hinge end and there was concern that they would buckle without some reinforcement. Once the second brace was added, it was believed that the crease could be deleted, so they used both styles in parallel for some period of time. Once it was determined that the crease wasn't necessary, Jensen and the dealers were instructed to use up the creased bonnets they had in inventory. This explained (to me at least) why the bonnets aren't found in any consistent body number sequence like other running changes usually are.

So we have two differing explanations, each from one of the two people who were intimately involved with the car. While I personally would give higher odds to Geoff's explanation, I don't think there is any way we will ever be sure.