It's a well known psychological ploy.
https://www.psychologicalscience.org/news/minds-business/to-be-more-persuasive-repeat-yourself.html
It'll work too, because by saying "strong and stable" over and over again, without deviation, you're showing
consistency, which is in itself "strong and stable".
She
wants to be boring; that's her whole point. Dull but dependable. That's the mindset she wants the voters to have on her.
It means that when Labour do something utterly stupid, which given McDonnell, Thornberry, Abbott and Corbyn are at the heart of Labour currently means you won't have to go searching for stupid things, then by contrast the Tories look calm and rational.
You may dislike the tactic, but it's why she's going to win by a landslide. She doesn't need to campaign with style or even substance - she just has to make few mistakes.