Hi Graeme,
It’s because in the pages you’ve set the GB container’s inner width to 1000px, while the page’s content container width is 1200px (the setting is at customizer > layout > container).
For the posts, you are using a block element built with GB blocks, so the inner container width matches with the rest containers.
Solution 1: You can try this CSS:
.page h1.entry-title {
max-width: 1000px;
margin-left: auto;
margin-right: auto;
}
Solution 2: You can create a block element – page hero for the pages as well, just like the posts.