There is a lot of variety when it comes to International Phonetic Alphabet (IPA) systems for any given language or dialect because the IPA symbols aren't extremely precise. Historically, some publishers chose an IPA that was less accurate but more readable, while other publishers chose an IPA that was more accurate but less readable.
We decided to go with an IPA that is more accurate than the average. For Received Pronunciation, we're pretty similar to Oxford's transcriptions here:
I was tempted to go with CUBE's transcriptions, but my linguist suggested that they're so weird that I'd confuse a lot of students
Any changes to Oxford have to do with us getting a little more accurate.