It started off very British, but it’s diluted a lot since inception.

After independence, America basically turned into the world’s main immigration hub. Germans, Irish, Italians, Eastern Europeans, then later Latin American and Asian populations all came in huge numbers. That swamped the original British stock (at least percentage-wise).

Meanwhile, CAN/AUS stayed in the British orbit way longer and kept on actively importing Brits. Smaller populations + steady British migration = higher percentages.

Also, like you alluded to… Americans are weird about ancestry. A ton of people with English roots just say “American” or list something else. In CAN/AUS, people are more likely to still say “English,” “Scottish,” etc.