With the exception of Dukes all other nobles in the UK are generally styled as lord xxx instead of Baron, Earl, Marquess xxx.
Princess Maud like any other sibling heir or heiress was not styled with a courtesy title. That is only done for the oldest son (there might be a few Scottish examples where a daughter is styled with it) and his oldest son if there is another courtesy title available.
So was this the courtesy title of Maud's husband from 1905 (when his father became the 10th Earl of Southesk) until 1941, when he himself succeeded as the 11th? According to Wikipedia, he was styled as Lord Charles Carnegie during those years. Indeed, I believe that when the heir to a peerage assumes a courtesy title of Baron or Viscount, he is oftentimes styled simply as LORD for simplicity.
The present Earl of Saint Andrews (his courtesy title from birth, as the elder son and heir to the Duke of Kent), for instance, has only one son, who from birth has been styled as Lord Downpatrick. I believe that the actual tertiary title of the Kent dukedom is BARON Downpatrick.
By this logic should not Charles Carnegie, then, been styled as Lord Balinhard? Or perhaps he didn't use the secondary title of the Southesk earldom for courtesy, choosing instead to be known simply as LORD? Certainly as the eldest son and heir of an earl, he was entitled to style himself higher than a mere Honourable ...
As for Maud's son: I suppose he would have had no choice but to style himself simply as The Honourable James Carnegie, during the first 12 years of his life. After all, his father was not yet the 11th Earl of Southesk, and any child of a baron (like a child of a viscount) is only Honourable. Of course, as the only son of the eldest son of an earl, he was due to eventually inherit a peerage. As it was, his grandfather died in 1941: I'm assuming that James inherited the courtesy title of Baron Balinhard (previously held by his father) on the occasion, when becoming the new heir. Was he then styled as Lord James Carnegie, just as his father had been styled as Lord Charles Carnegie during his own years as heir?
And what happened two years later, when his cousin Alistair (who had been the heir to two dukedoms) died, making his mother into the new heir to Fife? It has been stated that Maud was never styled by the courtesy title Countess of Macduff (there really was no need, since she by then was the official Countess of Southesk). Did James get the title Earl of Macduff, at the age of 14, when he found himself second in the succession to the Fife dukedom?
There is a secondary title associated with the earldom of Southesk: Baron Balinhard